"""
Copyright 2020 The OneFlow Authors. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
"""

import math
import warnings
import numbers
from typing import List, Tuple, Optional

import oneflow as flow
from oneflow import nn
from oneflow.framework.tensor import Tensor
from oneflow.nn.utils.rnn import PackedSequence

# NOTE(Liang Depeng): The implementation of rnn modules are modified from
#                     https://github.com/pytorch/pytorch/blob/master/torch/nn/modules/rnn.py
def apply_permutation(tensor: Tensor, permutation: Tensor, dim: int = 1) -> Tensor:
    return tensor.index_select(dim, permutation)


class RNNBase(nn.Module):
    def __init__(
        self,
        mode: str,
        input_size: int,
        hidden_size: int,
        num_layers: int = 1,
        bias: bool = True,
        batch_first: bool = False,
        dropout: float = 0.0,
        bidirectional: bool = False,
        proj_size: int = 0,
        device=None,
        dtype=None,
    ) -> None:
        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__()
        self.mode = mode
        self.input_size = input_size
        self.hidden_size = hidden_size
        self.num_layers = num_layers
        self.bias = bias
        self.batch_first = batch_first
        self.dropout = float(dropout)
        self.bidirectional = bidirectional
        self.proj_size = proj_size
        num_directions = 2 if bidirectional else 1

        if (
            not isinstance(dropout, numbers.Number)
            or not 0 <= dropout <= 1
            or isinstance(dropout, bool)
        ):
            raise ValueError(
                "dropout should be a number in range [0, 1] "
                "representing the probability of an element being "
                "zeroed"
            )
        if dropout > 0 and num_layers == 1:
            warnings.warn(
                "dropout option adds dropout after all but last "
                "recurrent layer, so non-zero dropout expects "
                "num_layers greater than 1, but got dropout={} and "
                "num_layers={}".format(dropout, num_layers)
            )
        if proj_size < 0:
            raise ValueError(
                "proj_size should be a positive integer or zero to disable projections"
            )
        if proj_size >= hidden_size:
            raise ValueError("proj_size has to be smaller than hidden_size")

        if mode == "LSTM":
            gate_size = 4 * hidden_size
        elif mode == "GRU":
            gate_size = 3 * hidden_size
        elif mode == "RNN_TANH":
            gate_size = hidden_size
        elif mode == "RNN_RELU":
            gate_size = hidden_size
        else:
            raise ValueError("Unrecognized RNN mode: " + mode)

        self._flat_weights_names = []
        self._all_weights = []
        for layer in range(num_layers):
            for direction in range(num_directions):
                real_hidden_size = proj_size if proj_size > 0 else hidden_size
                layer_input_size = (
                    input_size if layer == 0 else real_hidden_size * num_directions
                )

                w_ih = nn.Parameter(
                    flow.empty((gate_size, layer_input_size), **factory_kwargs)
                )
                w_hh = nn.Parameter(
                    flow.empty((gate_size, real_hidden_size), **factory_kwargs)
                )
                b_ih = nn.Parameter(flow.empty(gate_size, **factory_kwargs))
                b_hh = nn.Parameter(flow.empty(gate_size, **factory_kwargs))
                layer_params: Tuple[Tensor, ...] = ()
                if self.proj_size == 0:
                    if bias:
                        layer_params = (w_ih, w_hh, b_ih, b_hh)
                    else:
                        layer_params = (w_ih, w_hh)
                else:
                    w_hr = nn.Parameter(
                        flow.empty((proj_size, hidden_size), **factory_kwargs)
                    )
                    if bias:
                        layer_params = (w_ih, w_hh, b_ih, b_hh, w_hr)
                    else:
                        layer_params = (w_ih, w_hh, w_hr)

                suffix = "_reverse" if direction == 1 else ""
                param_names = ["weight_ih_l{}{}", "weight_hh_l{}{}"]
                if bias:
                    param_names += ["bias_ih_l{}{}", "bias_hh_l{}{}"]
                if self.proj_size > 0:
                    param_names += ["weight_hr_l{}{}"]
                param_names = [x.format(layer, suffix) for x in param_names]

                for name, param in zip(param_names, layer_params):
                    setattr(self, name, param)
                self._flat_weights_names.extend(param_names)
                self._all_weights.append(param_names)

        self._flat_weights = [
            (lambda wn: getattr(self, wn) if hasattr(self, wn) else None)(wn)
            for wn in self._flat_weights_names
        ]
        self.reset_parameters()

    def __setattr__(self, attr, value):
        if hasattr(self, "_flat_weights_names") and attr in self._flat_weights_names:
            # keep self._flat_weights up to date if you do self.weight = ...
            idx = self._flat_weights_names.index(attr)
            self._flat_weights[idx] = value
        super().__setattr__(attr, value)

    def to_global(self, placement=None, sbp=None):
        def convert(t):
            return t.to_global(placement=placement, sbp=sbp)

        self = self._apply(convert)
        self._flat_weights = [
            (lambda wn: getattr(self, wn) if hasattr(self, wn) else None)(wn)
            for wn in self._flat_weights_names
        ]
        return self

    def reset_parameters(self) -> None:
        stdv = 1.0 / math.sqrt(self.hidden_size) if self.hidden_size > 0 else 0
        for weight in self.parameters():
            nn.init.uniform_(weight, -stdv, stdv)

    def check_input(self, input: Tensor, batch_sizes: Optional[Tensor]) -> None:
        expected_input_dim = 2 if batch_sizes is not None else 3
        if input.dim() != expected_input_dim:
            raise RuntimeError(
                "input must have {} dimensions, got {}".format(
                    expected_input_dim, input.dim()
                )
            )
        if self.input_size != input.size(-1):
            raise RuntimeError(
                "input.size(-1) must be equal to input_size. Expected {}, got {}".format(
                    self.input_size, input.size(-1)
                )
            )

    def get_expected_hidden_size(
        self, input: Tensor, batch_sizes: Optional[Tensor]
    ) -> Tuple[int, int, int]:
        if batch_sizes is not None:
            mini_batch = int(batch_sizes[0])
        else:
            mini_batch = input.size(0) if self.batch_first else input.size(1)
        num_directions = 2 if self.bidirectional else 1
        if self.proj_size > 0:
            expected_hidden_size = (
                self.num_layers * num_directions,
                mini_batch,
                self.proj_size,
            )
        else:
            expected_hidden_size = (
                self.num_layers * num_directions,
                mini_batch,
                self.hidden_size,
            )
        return expected_hidden_size

    def check_hidden_size(
        self,
        hx: Tensor,
        expected_hidden_size: Tuple[int, int, int],
        msg: str = "Expected hidden size {}, got {}",
    ) -> None:
        if hx.size() != expected_hidden_size:
            raise RuntimeError(msg.format(expected_hidden_size, list(hx.size())))

    def check_forward_args(
        self, input: Tensor, hidden: Tensor, batch_sizes: Optional[Tensor]
    ):
        self.check_input(input, batch_sizes)
        expected_hidden_size = self.get_expected_hidden_size(input, batch_sizes)

        self.check_hidden_size(hidden, expected_hidden_size)

    def permute_hidden(self, hx: Tensor, permutation: Optional[Tensor]):
        if permutation is None:
            return hx
        return apply_permutation(hx, permutation)

    def extra_repr(self) -> str:
        s = "{input_size}, {hidden_size}"
        if self.proj_size != 0:
            s += ", proj_size={proj_size}"
        if self.num_layers != 1:
            s += ", num_layers={num_layers}"
        if self.bias is not True:
            s += ", bias={bias}"
        if self.batch_first is not False:
            s += ", batch_first={batch_first}"
        if self.dropout != 0:
            s += ", dropout={dropout}"
        if self.bidirectional is not False:
            s += ", bidirectional={bidirectional}"
        return s.format(**self.__dict__)

    @property
    def all_weights(self) -> List[List[nn.Parameter]]:
        return [
            [getattr(self, weight) for weight in weights]
            for weights in self._all_weights
        ]


class RNN(RNNBase):
    """The interface is consistent with PyTorch.
    The documentation is referenced from: https://pytorch.org/docs/1.10/generated/torch.nn.RNN.html.

    Applies a multi-layer Elman RNN with \tanhtanh or \text{ReLU}ReLU non-linearity to an input sequence.

    For each element in the input sequence, each layer computes the following function:
    
    function:

    .. math::
        h_t = \tanh(W_{ih} x_t + b_{ih} + W_{hh} h_{(t-1)} + b_{hh})

    where :math:`h_t` is the hidden state at time `t`, :math:`x_t` is
    the input at time `t`, and :math:`h_{(t-1)}` is the hidden state of the
    previous layer at time `t-1` or the initial hidden state at time `0`.
    If :attr:`nonlinearity` is ``'relu'``, then :math:`\text{ReLU}` is used instead of :math:`\tanh`.

    Args:
        input_size: The number of expected features in the input `x`
        hidden_size: The number of features in the hidden state `h`
        num_layers: Number of recurrent layers. E.g., setting ``num_layers=2``
            would mean stacking two RNNs together to form a `stacked RNN`,
            with the second RNN taking in outputs of the first RNN and
            computing the final results. Default: 1
        nonlinearity: The non-linearity to use. Can be either ``'tanh'`` or ``'relu'``. Default: ``'tanh'``
        bias: If ``False``, then the layer does not use bias weights `b_ih` and `b_hh`.
            Default: ``True``
        batch_first: If ``True``, then the input and output tensors are provided
            as `(batch, seq, feature)` instead of `(seq, batch, feature)`.
            Note that this does not apply to hidden or cell states. See the
            Inputs/Outputs sections below for details.  Default: ``False``
        dropout: If non-zero, introduces a `Dropout` layer on the outputs of each
            RNN layer except the last layer, with dropout probability equal to
            :attr:`dropout`. Default: 0
        bidirectional: If ``True``, becomes a bidirectional RNN. Default: ``False``

    Inputs: input, h_0
        * **input**: tensor of shape :math:`(L, N, H_{in})` when ``batch_first=False`` or
          :math:`(N, L, H_{in})` when ``batch_first=True`` containing the features of
          the input sequence.
        * **h_0**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{out})` containing the initial hidden
          state for each element in the batch. Defaults to zeros if not provided.

        where:
        .. math::
            \begin{aligned}
                N ={} & \text{batch size} \\
                L ={} & \text{sequence length} \\
                D ={} & 2 \text{ if bidirectional=True otherwise } 1 \\
                H_{in} ={} & \text{input\_size} \\
                H_{out} ={} & \text{hidden\_size}
            \end{aligned}

    Outputs: output, h_n
        * **output**: tensor of shape :math:`(L, N, D * H_{out})` when ``batch_first=False`` or
          :math:`(N, L, D * H_{out})` when ``batch_first=True`` containing the output features
          `(h_t)` from the last layer of the RNN, for each `t`.
        * **h_n**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{out})` containing the final hidden state
          for each element in the batch.

    Attributes:
        weight_ih_l[k]: the learnable input-hidden weights of the k-th layer,
            of shape `(hidden_size, input_size)` for `k = 0`. Otherwise, the shape is
            `(hidden_size, num_directions * hidden_size)`
        weight_hh_l[k]: the learnable hidden-hidden weights of the k-th layer,
            of shape `(hidden_size, hidden_size)`
        bias_ih_l[k]: the learnable input-hidden bias of the k-th layer,
            of shape `(hidden_size)`
        bias_hh_l[k]: the learnable hidden-hidden bias of the k-th layer,
            of shape `(hidden_size)`

    .. note::
        All the weights and biases are initialized from :math:`\mathcal{U}(-\sqrt{k}, \sqrt{k})`
        where :math:`k = \frac{1}{\text{hidden\_size}}`
    
    .. note::
        For bidirectional RNNs, forward and backward are directions 0 and 1 respectively.
        Example of splitting the output layers when ``batch_first=False``:
        ``output.view((seq_len, batch, num_directions, hidden_size))``.
    
    For example:

    .. code-block:: python

        >>> import oneflow as flow
        >>> import numpy as np
        >>> rnn = flow.nn.RNN(10, 20, 2)
        >>> input = flow.tensor(np.random.randn(5, 3, 10), dtype=flow.float32)
        >>> h0 = flow.tensor(np.random.randn(2, 3, 20), dtype=flow.float32)
        >>> output, hn = rnn(input, h0)
        >>> output.size()
        oneflow.Size([5, 3, 20])

    """

    def __init__(self, *args, **kwargs):
        if "proj_size" in kwargs:
            raise ValueError(
                "proj_size argument is only supported for LSTM, not RNN or GRU"
            )
        self.nonlinearity = kwargs.pop("nonlinearity", "tanh")
        if self.nonlinearity == "tanh":
            mode = "RNN_TANH"
        elif self.nonlinearity == "relu":
            mode = "RNN_RELU"
        else:
            raise ValueError("Unknown nonlinearity '{}'".format(self.nonlinearity))
        super().__init__(mode, *args, **kwargs)

    def forward(self, input, hx=None):  # noqa: F811
        orig_input = input
        if isinstance(orig_input, PackedSequence):
            input = orig_input.data
            batch_sizes = orig_input.batch_sizes
            sorted_indices = orig_input.sorted_indices
            unsorted_indices = orig_input.unsorted_indices
            max_batch_size = int(batch_sizes[0])
        else:
            batch_sizes = None
            is_batched = input.dim() == 3
            batch_dim = 0 if self.batch_first else 1
            if not is_batched:
                input = input.unsqueeze(batch_dim)
                if hx is not None:
                    if hx.dim() != 2:
                        raise RuntimeError(
                            f"For unbatched 2-D input, hx should also be 2-D but got {hx.dim()}-D tensor"
                        )
                    hx = hx.unsqueeze(1)
            else:
                if hx is not None and hx.dim() != 3:
                    raise RuntimeError(
                        f"For batched 3-D input, hx should also be 3-D but got {hx.dim()}-D tensor"
                    )
            max_batch_size = input.size(0) if self.batch_first else input.size(1)
            sorted_indices = None
            unsorted_indices = None

        if hx is None:
            num_directions = 2 if self.bidirectional else 1
            if input.is_global:
                hx = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    self.hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
            else:
                hx = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    self.hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
        else:
            # Each batch of the hidden state should match the input sequence that
            # the user believes he/she is passing in.
            hx = self.permute_hidden(hx, sorted_indices)
        self._flat_weights = [
            (lambda wn: getattr(self, wn) if hasattr(self, wn) else None)(wn)
            for wn in self._flat_weights_names
        ]
        assert hx is not None
        self.check_forward_args(input, hx, batch_sizes)
        assert self.mode == "RNN_TANH" or self.mode == "RNN_RELU"
        if batch_sizes is None:
            if self.mode == "RNN_TANH":
                result = flow._C.rnn_tanh(
                    input,
                    hx,
                    self._flat_weights,
                    self.bias,
                    self.num_layers,
                    self.dropout,
                    self.training,
                    self.bidirectional,
                    self.batch_first,
                )
            else:
                result = flow._C.rnn_relu(
                    input,
                    hx,
                    self._flat_weights,
                    self.bias,
                    self.num_layers,
                    self.dropout,
                    self.training,
                    self.bidirectional,
                    self.batch_first,
                )
        else:
            if self.mode == "RNN_TANH":
                result = flow._C.rnn_tanh(
                    input,
                    batch_sizes,
                    hx,
                    self._flat_weights,
                    self.bias,
                    self.num_layers,
                    self.dropout,
                    self.training,
                    self.bidirectional,
                )
            else:
                result = flow._C.rnn_relu(
                    input,
                    batch_sizes,
                    hx,
                    self._flat_weights,
                    self.bias,
                    self.num_layers,
                    self.dropout,
                    self.training,
                    self.bidirectional,
                )

        output = result[0]
        hidden = result[1]

        if isinstance(orig_input, PackedSequence):
            output_packed = PackedSequence(
                output, batch_sizes, sorted_indices, unsorted_indices
            )
            return output_packed, self.permute_hidden(hidden, unsorted_indices)

        if not is_batched:
            output = output.squeeze(batch_dim)
            hidden = hidden.squeeze(1)

        return output, self.permute_hidden(hidden, unsorted_indices)


class LSTM(RNNBase):
    """The interface is consistent with PyTorch.
    The documentation is referenced from: https://pytorch.org/docs/1.10/_modules/torch/nn/modules/rnn.html#LSTM.

    Applies a multi-layer long short-term memory (LSTM) RNN to an input sequence.

    For each element in the input sequence, each layer computes the following
    
    function:

    .. math::
        \begin{array}{ll} \\
            i_t = \sigma(W_{ii} x_t + b_{ii} + W_{hi} h_{t-1} + b_{hi}) \\
            f_t = \sigma(W_{if} x_t + b_{if} + W_{hf} h_{t-1} + b_{hf}) \\
            g_t = \tanh(W_{ig} x_t + b_{ig} + W_{hg} h_{t-1} + b_{hg}) \\
            o_t = \sigma(W_{io} x_t + b_{io} + W_{ho} h_{t-1} + b_{ho}) \\
            c_t = f_t \odot c_{t-1} + i_t \odot g_t \\
            h_t = o_t \odot \tanh(c_t) \\
        \end{array}

    where :math:`h_t` is the hidden state at time `t`, :math:`c_t` is the cell
    state at time `t`, :math:`x_t` is the input at time `t`, :math:`h_{t-1}`
    is the hidden state of the layer at time `t-1` or the initial hidden
    state at time `0`, and :math:`i_t`, :math:`f_t`, :math:`g_t`,
    :math:`o_t` are the input, forget, cell, and output gates, respectively.
    :math:`\sigma` is the sigmoid function, and :math:`\odot` is the Hadamard product.

    In a multilayer LSTM, the input :math:`x^{(l)}_t` of the :math:`l` -th layer
    (:math:`l >= 2`) is the hidden state :math:`h^{(l-1)}_t` of the previous layer multiplied by
    dropout :math:`\delta^{(l-1)}_t` where each :math:`\delta^{(l-1)}_t` is a Bernoulli random
    variable which is :math:`0` with probability :attr:`dropout`.

    If ``proj_size > 0`` is specified, LSTM with projections will be used. This changes
    the LSTM cell in the following way. First, the dimension of :math:`h_t` will be changed from
    ``hidden_size`` to ``proj_size`` (dimensions of :math:`W_{hi}` will be changed accordingly).
    Second, the output hidden state of each layer will be multiplied by a learnable projection
    matrix: :math:`h_t = W_{hr}h_t`. Note that as a consequence of this, the output
    of LSTM network will be of different shape as well. See Inputs/Outputs sections below for exact
    dimensions of all variables. You can find more details in https://arxiv.org/abs/1402.1128.

    Args:
        input_size: The number of expected features in the input `x`
        hidden_size: The number of features in the hidden state `h`
        num_layers: Number of recurrent layers. E.g., setting ``num_layers=2``
            would mean stacking two LSTMs together to form a `stacked LSTM`,
            with the second LSTM taking in outputs of the first LSTM and
            computing the final results. Default: 1
        bias: If ``False``, then the layer does not use bias weights `b_ih` and `b_hh`.
            Default: ``True``
        batch_first: If ``True``, then the input and output tensors are provided
            as `(batch, seq, feature)` instead of `(seq, batch, feature)`.
            Note that this does not apply to hidden or cell states. See the
            Inputs/Outputs sections below for details.  Default: ``False``
        dropout: If non-zero, introduces a `Dropout` layer on the outputs of each
            LSTM layer except the last layer, with dropout probability equal to
            :attr:`dropout`. Default: 0
        bidirectional: If ``True``, becomes a bidirectional LSTM. Default: ``False``
        proj_size: If ``> 0``, will use LSTM with projections of corresponding size. Default: 0

    Inputs: input, (h_0, c_0)
        * **input**: tensor of shape :math:`(L, N, H_{in})` when ``batch_first=False`` or
          :math:`(N, L, H_{in})` when ``batch_first=True`` containing the features of
          the input sequence.
        * **h_0**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{out})` containing the
          initial hidden state for each element in the batch.
          Defaults to zeros if (h_0, c_0) is not provided.
        * **c_0**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{cell})` containing the
          initial cell state for each element in the batch.
          Defaults to zeros if (h_0, c_0) is not provided.

        where:

        .. math::
            \begin{aligned}
                N ={} & \text{batch size} \\
                L ={} & \text{sequence length} \\
                D ={} & 2 \text{ if bidirectional=True otherwise } 1 \\
                H_{in} ={} & \text{input\_size} \\
                H_{cell} ={} & \text{hidden\_size} \\
                H_{out} ={} & \text{proj\_size if } \text{proj\_size}>0 \text{ otherwise hidden\_size} \\
            \end{aligned}

    Outputs: output, (h_n, c_n)
        * **output**: tensor of shape :math:`(L, N, D * H_{out})` when ``batch_first=False`` or
          :math:`(N, L, D * H_{out})` when ``batch_first=True`` containing the output features
          `(h_t)` from the last layer of the LSTM, for each `t`.
        * **h_n**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{out})` containing the
          final hidden state for each element in the batch.
        * **c_n**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{cell})` containing the
          final cell state for each element in the batch.

    Attributes:
        weight_ih_l[k] : the learnable input-hidden weights of the :math:`\text{k}^{th}` layer
            `(W_ii|W_if|W_ig|W_io)`, of shape `(4*hidden_size, input_size)` for `k = 0`.
            Otherwise, the shape is `(4*hidden_size, num_directions * hidden_size)`
        weight_hh_l[k] : the learnable hidden-hidden weights of the :math:`\text{k}^{th}` layer
            `(W_hi|W_hf|W_hg|W_ho)`, of shape `(4*hidden_size, hidden_size)`. If ``proj_size > 0``
            was specified, the shape will be `(4*hidden_size, proj_size)`.
        bias_ih_l[k] : the learnable input-hidden bias of the :math:`\text{k}^{th}` layer
            `(b_ii|b_if|b_ig|b_io)`, of shape `(4*hidden_size)`
        bias_hh_l[k] : the learnable hidden-hidden bias of the :math:`\text{k}^{th}` layer
            `(b_hi|b_hf|b_hg|b_ho)`, of shape `(4*hidden_size)`
        weight_hr_l[k] : the learnable projection weights of the :math:`\text{k}^{th}` layer
            of shape `(proj_size, hidden_size)`. Only present when ``proj_size > 0`` was
            specified.

    .. note::
        All the weights and biases are initialized from :math:`\mathcal{U}(-\sqrt{k}, \sqrt{k})`
        where :math:`k = \frac{1}{\text{hidden\_size}}`

    .. note::
        For bidirectional LSTMs, forward and backward are directions 0 and 1 respectively.
        Example of splitting the output layers when ``batch_first=False``:
        ``output.view(seq_len, batch, num_directions, hidden_size)``.

    For example:

    .. code-block:: python

        >>> import oneflow as flow
        >>> import numpy as np
        >>> rnn = flow.nn.LSTM(10, 20, 2)
        >>> input = flow.tensor(np.random.randn(5, 3, 10), dtype=flow.float32)
        >>> h0 = flow.tensor(np.random.randn(2, 3, 20), dtype=flow.float32)
        >>> c0 = flow.tensor(np.random.randn(2, 3, 20), dtype=flow.float32)
        >>> output, (hn, cn) = rnn(input, (h0, c0))
        >>> output.size()
        oneflow.Size([5, 3, 20])
        
    """

    def __init__(self, *args, **kwargs):
        super().__init__("LSTM", *args, **kwargs)

    def get_expected_cell_size(
        self, input: Tensor, batch_sizes: Optional[Tensor]
    ) -> Tuple[int, int, int]:
        if batch_sizes is not None:
            mini_batch = int(batch_sizes[0])
        else:
            mini_batch = input.size(0) if self.batch_first else input.size(1)
        num_directions = 2 if self.bidirectional else 1
        expected_hidden_size = (
            self.num_layers * num_directions,
            mini_batch,
            self.hidden_size,
        )
        return expected_hidden_size

    def check_forward_args(
        self,
        input: Tensor,
        hidden: Tuple[Tensor, Tensor],
        batch_sizes: Optional[Tensor],
    ):
        self.check_input(input, batch_sizes)
        self.check_hidden_size(
            hidden[0],
            self.get_expected_hidden_size(input, batch_sizes),
            "Expected hidden[0] size {}, got {}",
        )
        self.check_hidden_size(
            hidden[1],
            self.get_expected_cell_size(input, batch_sizes),
            "Expected hidden[1] size {}, got {}",
        )

    def permute_hidden(
        self, hx: Tuple[Tensor, Tensor], permutation: Optional[Tensor]
    ) -> Tuple[Tensor, Tensor]:
        if permutation is None:
            return hx
        return (
            apply_permutation(hx[0], permutation),
            apply_permutation(hx[1], permutation),
        )

    def forward(self, input, hx=None):
        orig_input = input
        batch_sizes = None
        if isinstance(orig_input, PackedSequence):
            input = orig_input.data
            batch_sizes = orig_input.batch_sizes
            sorted_indices = orig_input.sorted_indices
            unsorted_indices = orig_input.unsorted_indices
            max_batch_size = int(batch_sizes[0])
        else:
            batch_sizes = None
            is_batched = input.dim() == 3
            batch_dim = 0 if self.batch_first else 1
            if not is_batched:
                input = input.unsqueeze(batch_dim)
            max_batch_size = input.size(0) if self.batch_first else input.size(1)
            sorted_indices = None
            unsorted_indices = None

        if hx is None:
            num_directions = 2 if self.bidirectional else 1
            real_hidden_size = (
                self.proj_size if self.proj_size > 0 else self.hidden_size
            )

            if input.is_global:
                h_zeros = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    real_hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
                c_zeros = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    self.hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
            else:
                h_zeros = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    real_hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
                c_zeros = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    self.hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
            hx = (h_zeros, c_zeros)
        else:
            if batch_sizes is None:  # If not PackedSequence input.
                if is_batched:
                    if hx[0].dim() != 3 or hx[1].dim() != 3:
                        msg = (
                            "For batched 3-D input, hx and cx should "
                            f"also be 3-D but got ({hx[0].dim()}-D, {hx[1].dim()}-D) tensors"
                        )
                        raise RuntimeError(msg)
                else:
                    if hx[0].dim() != 2 or hx[1].dim() != 2:
                        msg = (
                            "For unbatched 2-D input, hx and cx should "
                            f"also be 2-D but got ({hx[0].dim()}-D, {hx[1].dim()}-D) tensors"
                        )
                        raise RuntimeError(msg)
                    hx = (hx[0].unsqueeze(1), hx[1].unsqueeze(1))

            # Each batch of the hidden state should match the input sequence that
            # the user believes he/she is passing in.
            hx = self.permute_hidden(hx, sorted_indices)

        self.check_forward_args(input, hx, batch_sizes)
        self._flat_weights = [
            (lambda wn: getattr(self, wn) if hasattr(self, wn) else None)(wn)
            for wn in self._flat_weights_names
        ]
        if batch_sizes is None:
            result = flow._C.lstm(
                input,
                hx,
                self._flat_weights,
                self.bias,
                self.num_layers,
                self.dropout,
                self.training,
                self.bidirectional,
                self.batch_first,
            )
        else:
            result = flow._C.lstm(
                input,
                batch_sizes,
                hx,
                self._flat_weights,
                self.bias,
                self.num_layers,
                self.dropout,
                self.training,
                self.bidirectional,
            )
        output = result[0]
        hidden = result[1:]
        if isinstance(orig_input, PackedSequence):
            output_packed = PackedSequence(
                output, batch_sizes, sorted_indices, unsorted_indices
            )
            return output_packed, self.permute_hidden(hidden, unsorted_indices)
        else:
            if not is_batched:
                output = output.squeeze(batch_dim)
                hidden = (hidden[0].squeeze(1), hidden[1].squeeze(1))
            return output, self.permute_hidden(hidden, unsorted_indices)


class GRU(RNNBase):
    """The interface is consistent with PyTorch.
    The documentation is referenced from: https://pytorch.org/docs/1.10/_modules/torch/nn/modules/rnn.html#GRU.

    Applies a multi-layer gated recurrent unit (GRU) RNN to an input sequence.

    For each element in the input sequence, each layer computes the following
    
    function:

    .. math::
        \begin{array}{ll}
            r_t = \sigma(W_{ir} x_t + b_{ir} + W_{hr} h_{(t-1)} + b_{hr}) \\
            z_t = \sigma(W_{iz} x_t + b_{iz} + W_{hz} h_{(t-1)} + b_{hz}) \\
            n_t = \tanh(W_{in} x_t + b_{in} + r_t * (W_{hn} h_{(t-1)}+ b_{hn})) \\
            h_t = (1 - z_t) * n_t + z_t * h_{(t-1)}
        \end{array}
    
    where :math:`h_t` is the hidden state at time `t`, :math:`x_t` is the input
    at time `t`, :math:`h_{(t-1)}` is the hidden state of the layer
    at time `t-1` or the initial hidden state at time `0`, and :math:`r_t`,
    :math:`z_t`, :math:`n_t` are the reset, update, and new gates, respectively.
    :math:`\sigma` is the sigmoid function, and :math:`*` is the Hadamard product.

    In a multilayer GRU, the input :math:`x^{(l)}_t` of the :math:`l` -th layer
    (:math:`l >= 2`) is the hidden state :math:`h^{(l-1)}_t` of the previous layer multiplied by
    dropout :math:`\delta^{(l-1)}_t` where each :math:`\delta^{(l-1)}_t` is a Bernoulli random
    variable which is :math:`0` with probability :attr:`dropout`.

    Args:
        input_size: The number of expected features in the input `x`
        hidden_size: The number of features in the hidden state `h`
        num_layers: Number of recurrent layers. E.g., setting ``num_layers=2``
            would mean stacking two GRUs together to form a `stacked GRU`,
            with the second GRU taking in outputs of the first GRU and
            computing the final results. Default: 1
        bias: If ``False``, then the layer does not use bias weights `b_ih` and `b_hh`.
            Default: ``True``
        batch_first: If ``True``, then the input and output tensors are provided
            as `(batch, seq, feature)` instead of `(seq, batch, feature)`.
            Note that this does not apply to hidden or cell states. See the
            Inputs/Outputs sections below for details.  Default: ``False``
        dropout: If non-zero, introduces a `Dropout` layer on the outputs of each
            GRU layer except the last layer, with dropout probability equal to
            :attr:`dropout`. Default: 0
        bidirectional: If ``True``, becomes a bidirectional GRU. Default: ``False``

    Inputs: input, h_0
        * **input**: tensor of shape :math:`(L, N, H_{in})` when ``batch_first=False`` or
          :math:`(N, L, H_{in})` when ``batch_first=True`` containing the features of
          the input sequence. 
        * **h_0**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{out})` containing the initial hidden
          state for each element in the batch. Defaults to zeros if not provided.

        where:

        .. math::
            \begin{aligned}
                N ={} & \text{batch size} \\
                L ={} & \text{sequence length} \\
                D ={} & 2 \text{ if bidirectional=True otherwise } 1 \\
                H_{in} ={} & \text{input\_size} \\
                H_{out} ={} & \text{hidden\_size}
            \end{aligned}

    Outputs: output, h_n
        * **output**: tensor of shape :math:`(L, N, D * H_{out})` when ``batch_first=False`` or
          :math:`(N, L, D * H_{out})` when ``batch_first=True`` containing the output features
          `(h_t)` from the last layer of the GRU, for each `t`. If a
        * **h_n**: tensor of shape :math:`(D * \text{num\_layers}, N, H_{out})` containing the final hidden state
          for each element in the batch.

    Attributes:
        weight_ih_l[k] : the learnable input-hidden weights of the :math:`\text{k}^{th}` layer
            (W_ir|W_iz|W_in), of shape `(3*hidden_size, input_size)` for `k = 0`.
            Otherwise, the shape is `(3*hidden_size, num_directions * hidden_size)`
        weight_hh_l[k] : the learnable hidden-hidden weights of the :math:`\text{k}^{th}` layer
            (W_hr|W_hz|W_hn), of shape `(3*hidden_size, hidden_size)`
        bias_ih_l[k] : the learnable input-hidden bias of the :math:`\text{k}^{th}` layer
            (b_ir|b_iz|b_in), of shape `(3*hidden_size)`
        bias_hh_l[k] : the learnable hidden-hidden bias of the :math:`\text{k}^{th}` layer
            (b_hr|b_hz|b_hn), of shape `(3*hidden_size)`

    .. note::
        All the weights and biases are initialized from :math:`\mathcal{U}(-\sqrt{k}, \sqrt{k})`
        where :math:`k = \frac{1}{\text{hidden\_size}}`

    .. note::
        For bidirectional GRUs, forward and backward are directions 0 and 1 respectively.
        Example of splitting the output layers when ``batch_first=False``:
        ``output.view(seq_len, batch, num_directions, hidden_size)``.

    For example:

    .. code-block:: python

        >>> import oneflow as flow
        >>> import numpy as np
        >>> rnn = flow.nn.GRU(10, 20, 2)
        >>> input = flow.tensor(np.random.randn(5, 3, 10), dtype=flow.float32)
        >>> h0 = flow.tensor(np.random.randn(2, 3, 20), dtype=flow.float32)
        >>> output, hn = rnn(input, h0)
        >>> output.size()
        oneflow.Size([5, 3, 20])
    
    """

    def __init__(self, *args, **kwargs):
        if "proj_size" in kwargs:
            raise ValueError(
                "proj_size argument is only supported for LSTM, not RNN or GRU"
            )
        super().__init__("GRU", *args, **kwargs)

    def forward(self, input, hx=None):
        orig_input = input
        if isinstance(orig_input, PackedSequence):
            input = orig_input.data
            batch_sizes = orig_input.batch_sizes
            sorted_indices = orig_input.sorted_indices
            unsorted_indices = orig_input.unsorted_indices
            max_batch_size = int(batch_sizes[0])
        else:
            batch_sizes = None
            is_batched = input.dim() == 3
            batch_dim = 0 if self.batch_first else 1
            if not is_batched:
                input = input.unsqueeze(batch_dim)
                if hx is not None:
                    if hx.dim() != 2:
                        raise RuntimeError(
                            f"For unbatched 2-D input, hx should also be 2-D but got {hx.dim()}-D tensor"
                        )
                    hx = hx.unsqueeze(1)
            else:
                if hx is not None and hx.dim() != 3:
                    raise RuntimeError(
                        f"For batched 3-D input, hx should also be 3-D but got {hx.dim()}-D tensor"
                    )
            max_batch_size = input.size(0) if self.batch_first else input.size(1)
            sorted_indices = None
            unsorted_indices = None

        if hx is None:
            num_directions = 2 if self.bidirectional else 1
            if input.is_global:
                hx = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    self.hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
            else:
                hx = flow.zeros(
                    self.num_layers * num_directions,
                    max_batch_size,
                    self.hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
        else:
            # Each batch of the hidden state should match the input sequence that
            # the user believes he/she is passing in.
            hx = self.permute_hidden(hx, sorted_indices)

        self.check_forward_args(input, hx, batch_sizes)
        self._flat_weights = [
            (lambda wn: getattr(self, wn) if hasattr(self, wn) else None)(wn)
            for wn in self._flat_weights_names
        ]
        if batch_sizes is None:
            result = flow._C.gru(
                input,
                hx,
                self._flat_weights,
                self.bias,
                self.num_layers,
                self.dropout,
                self.training,
                self.bidirectional,
                self.batch_first,
            )
        else:
            result = flow._C.gru(
                input,
                batch_sizes,
                hx,
                self._flat_weights,
                self.bias,
                self.num_layers,
                self.dropout,
                self.training,
                self.bidirectional,
            )
        output = result[0]
        hidden = result[1]

        if isinstance(orig_input, PackedSequence):
            output_packed = PackedSequence(
                output, batch_sizes, sorted_indices, unsorted_indices
            )
            return output_packed, self.permute_hidden(hidden, unsorted_indices)
        else:
            if not is_batched:
                output = output.squeeze(batch_dim)
                hidden = hidden.squeeze(1)

            return output, self.permute_hidden(hidden, unsorted_indices)


class RNNCellBase(nn.Module):
    def __init__(
        self,
        input_size: int,
        hidden_size: int,
        bias: bool,
        num_chunks: int,
        device=None,
        dtype=None,
    ):
        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__()
        self.input_size = input_size
        self.hidden_size = hidden_size
        self.bias = bias
        self.weight_ih = nn.Parameter(
            flow.empty(num_chunks * hidden_size, input_size, **factory_kwargs)
        )
        self.weight_hh = nn.Parameter(
            flow.empty(num_chunks * hidden_size, hidden_size, **factory_kwargs)
        )
        if bias:
            self.bias_ih = nn.Parameter(
                flow.empty(num_chunks * hidden_size, **factory_kwargs)
            )
            self.bias_hh = nn.Parameter(
                flow.empty(num_chunks * hidden_size, **factory_kwargs)
            )
        else:
            self.register_parameter("bias_ih", None)
            self.register_parameter("bias_hh", None)

        self.reset_parameters()

    def extra_repr(self) -> str:
        s = "{input_size}, {hidden_size}"
        if "bias" in self.__dict__ and self.bias is not True:
            s += ", bias={bias}"
        if "nonlinearity" in self.__dict__ and self.nonlinearity != "tanh":
            s += ", nonlinearity={nonlinearity}"
        return s.format(**self.__dict__)

    def reset_parameters(self) -> None:
        stdv = 1.0 / math.sqrt(self.hidden_size) if self.hidden_size > 0 else 0
        for weight in self.parameters():
            nn.init.uniform_(weight, -stdv, stdv)


class RNNCell(RNNCellBase):
    """The interface is consistent with PyTorch.
    The documentation is referenced from: https://pytorch.org/docs/stable/generated/torch.nn.RNNCell.html.
    
    An Elman RNN cell with tanh or ReLU non-linearity.

    .. math::

        h' = \tanh(W_{ih} x + b_{ih}  +  W_{hh} h + b_{hh})

    If :attr:`nonlinearity` is `'relu'`, then ReLU is used in place of tanh.

    Args:
        input_size: The number of expected features in the input `x`
        hidden_size: The number of features in the hidden state `h`
        bias: If ``False``, then the layer does not use bias weights `b_ih` and `b_hh`.
            Default: ``True``
        nonlinearity: The non-linearity to use. Can be either ``'tanh'`` or ``'relu'``. Default: ``'tanh'``

    Inputs: input, hidden
        - **input**: tensor containing input features
        - **hidden**: tensor containing the initial hidden state
          Defaults to zero if not provided.

    Outputs: h'
        - **h'** of shape `(batch, hidden_size)`: tensor containing the next hidden state
          for each element in the batch

    Shape:
        - input: :math:`(N, H_{in})` or :math:`(H_{in})` tensor containing input features where
          :math:`H_{in}` = `input_size`.
        - hidden: :math:`(N, H_{out})` or :math:`(H_{out})` tensor containing the initial hidden
          state where :math:`H_{out}` = `hidden_size`. Defaults to zero if not provided.
        - output: :math:`(N, H_{out})` or :math:`(H_{out})` tensor containing the next hidden state.

    Attributes:
        weight_ih: the learnable input-hidden weights, of shape
            `(hidden_size, input_size)`
        weight_hh: the learnable hidden-hidden weights, of shape
            `(hidden_size, hidden_size)`
        bias_ih: the learnable input-hidden bias, of shape `(hidden_size)`
        bias_hh: the learnable hidden-hidden bias, of shape `(hidden_size)`

    .. note::
        All the weights and biases are initialized from :math:`\mathcal{U}(-\sqrt{k}, \sqrt{k})`
        where :math:`k = \frac{1}{\text{hidden\_size}}`

    For example:

    .. code-block:: python

        >>> import oneflow as flow
        >>> import oneflow.nn as nn

        >>> rnn = nn.RNNCell(10, 20)
        >>> input = flow.randn(6, 3, 10)
        >>> hx = flow.randn(3, 20)
        >>> hx = rnn(input[0], hx)
        >>> hx.size()
        oneflow.Size([3, 20])
    """

    def __init__(
        self,
        input_size: int,
        hidden_size: int,
        bias: bool = True,
        nonlinearity: str = "tanh",
        device=None,
        dtype=None,
    ):
        factory_kwargs = {"device": device, "dtype": dtype}
        super(RNNCell, self).__init__(
            input_size, hidden_size, bias, num_chunks=1, **factory_kwargs
        )
        self.nonlinearity = nonlinearity

    def forward(self, input: Tensor, hx: Optional[Tensor] = None) -> Tensor:
        assert input.dim() in (
            1,
            2,
        ), f"RNNCell: Expected input to be 1-D or 2-D but received {input.dim()}-D tensor"
        is_batched = input.dim() == 2
        if not is_batched:
            input = input.unsqueeze(0)

        if hx is None:
            if input.is_global():
                hx = flow.zeros(
                    input.size(0),
                    self.hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
            else:
                hx = flow.zeros(
                    input.size(0),
                    self.hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
        else:
            hx = hx.unsqueeze(0) if not is_batched else hx

        if self.nonlinearity == "tanh":
            ret = flow._C.rnn_tanh_cell(
                input, hx, self.weight_ih, self.weight_hh, self.bias_ih, self.bias_hh,
            )
        elif self.nonlinearity == "relu":
            ret = flow._C.rnn_relu_cell(
                input, hx, self.weight_ih, self.weight_hh, self.bias_ih, self.bias_hh,
            )
        else:
            raise RuntimeError("Unknown nonlinearity: {}".format(self.nonlinearity))

        if not is_batched:
            ret = ret.squeeze(0)

        return ret


class LSTMCell(RNNCellBase):
    """The interface is consistent with PyTorch.
    The documentation is referenced from: https://pytorch.org/docs/stable/generated/torch.nn.LSTMCell.html.
    
    A long short-term memory (LSTM) cell.

    .. math::

        \begin{array}{ll}
        i = \sigma(W_{ii} x + b_{ii} + W_{hi} h + b_{hi}) \\
        f = \sigma(W_{if} x + b_{if} + W_{hf} h + b_{hf}) \\
        g = \tanh(W_{ig} x + b_{ig} + W_{hg} h + b_{hg}) \\
        o = \sigma(W_{io} x + b_{io} + W_{ho} h + b_{ho}) \\
        c' = f * c + i * g \\
        h' = o * \tanh(c') \\
        \end{array}

    where :math:`\sigma` is the sigmoid function, and :math:`*` is the Hadamard product.

    Args:
        input_size: The number of expected features in the input `x`
        hidden_size: The number of features in the hidden state `h`
        bias: If ``False``, then the layer does not use bias weights `b_ih` and
            `b_hh`. Default: ``True``

    Inputs: input, (h_0, c_0)
        - **input** of shape `(batch, input_size)` or `(input_size)`: tensor containing input features
        - **h_0** of shape `(batch, hidden_size)` or `(hidden_size)`: tensor containing the initial hidden state
        - **c_0** of shape `(batch, hidden_size)` or `(hidden_size)`: tensor containing the initial cell state

          If `(h_0, c_0)` is not provided, both **h_0** and **c_0** default to zero.

    Outputs: (h_1, c_1)
        - **h_1** of shape `(batch, hidden_size)` or `(hidden_size)`: tensor containing the next hidden state
        - **c_1** of shape `(batch, hidden_size)` or `(hidden_size)`: tensor containing the next cell state

    Attributes:
        weight_ih: the learnable input-hidden weights, of shape
            `(4*hidden_size, input_size)`
        weight_hh: the learnable hidden-hidden weights, of shape
            `(4*hidden_size, hidden_size)`
        bias_ih: the learnable input-hidden bias, of shape `(4*hidden_size)`
        bias_hh: the learnable hidden-hidden bias, of shape `(4*hidden_size)`

    .. note::
        All the weights and biases are initialized from :math:`\mathcal{U}(-\sqrt{k}, \sqrt{k})`
        where :math:`k = \frac{1}{\text{hidden\_size}}`

    For example:

    .. code-block:: python

        >>> import oneflow as flow
        >>> import oneflow.nn as nn

        >>> rnn = nn.LSTMCell(10, 20) # (input_size, hidden_size)
        >>> input = flow.randn(2, 3, 10) # (time_steps, batch, input_size)
        >>> hx = flow.randn(3, 20) # (batch, hidden_size)
        >>> cx = flow.randn(3, 20)
        >>> hx, cx = rnn(input[0], (hx, cx))
        >>> hx.size()
        oneflow.Size([3, 20])
    """

    def __init__(
        self,
        input_size: int,
        hidden_size: int,
        bias: bool = True,
        device=None,
        dtype=None,
    ) -> None:
        factory_kwargs = {"device": device, "dtype": dtype}
        super(LSTMCell, self).__init__(
            input_size, hidden_size, bias, num_chunks=4, **factory_kwargs
        )

    def forward(
        self, input: Tensor, hx: Optional[Tuple[Tensor, Tensor]] = None
    ) -> Tuple[Tensor, Tensor]:
        assert input.dim() in (
            1,
            2,
        ), f"LSTMCell: Expected input to be 1-D or 2-D but received {input.dim()}-D tensor"
        is_batched = input.dim() == 2
        if not is_batched:
            input = input.unsqueeze(0)

        if hx is None:
            if input.is_global():
                zeros = flow.zeros(
                    input.size(0),
                    self.hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
            else:
                zeros = flow.zeros(
                    input.size(0),
                    self.hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
            hx = (zeros, zeros)
        else:
            hx = (hx[0].unsqueeze(0), hx[1].unsqueeze(0)) if not is_batched else hx

        ret = flow._C.lstm_cell(
            input, hx, self.weight_ih, self.weight_hh, self.bias_ih, self.bias_hh,
        )

        if not is_batched:
            ret = (ret[0].squeeze(0), ret[1].squeeze(0))
        return ret


class GRUCell(RNNCellBase):
    """The interface is consistent with PyTorch.
    The documentation is referenced from: https://pytorch.org/docs/stable/generated/torch.nn.GRUCell.html.
    
    A gated recurrent unit (GRU) cell

    .. math::

        \begin{array}{ll}
        r = \sigma(W_{ir} x + b_{ir} + W_{hr} h + b_{hr}) \\
        z = \sigma(W_{iz} x + b_{iz} + W_{hz} h + b_{hz}) \\
        n = \tanh(W_{in} x + b_{in} + r * (W_{hn} h + b_{hn})) \\
        h' = (1 - z) * n + z * h
        \end{array}

    where :math:`\sigma` is the sigmoid function, and :math:`*` is the Hadamard product.

    Args:
        input_size: The number of expected features in the input `x`
        hidden_size: The number of features in the hidden state `h`
        bias: If ``False``, then the layer does not use bias weights `b_ih` and
            `b_hh`. Default: ``True``

    Inputs: input, hidden
        - **input** : tensor containing input features
        - **hidden** : tensor containing the initial hidden
          state for each element in the batch.
          Defaults to zero if not provided.

    Outputs: h'
        - **h'** : tensor containing the next hidden state
          for each element in the batch

    Shape:
        - input: :math:`(N, H_{in})` or :math:`(H_{in})` tensor containing input features where
          :math:`H_{in}` = `input_size`.
        - hidden: :math:`(N, H_{out})` or :math:`(H_{out})` tensor containing the initial hidden
          state where :math:`H_{out}` = `hidden_size`. Defaults to zero if not provided.
        - output: :math:`(N, H_{out})` or :math:`(H_{out})` tensor containing the next hidden state.

    Attributes:
        weight_ih: the learnable input-hidden weights, of shape
            `(3*hidden_size, input_size)`
        weight_hh: the learnable hidden-hidden weights, of shape
            `(3*hidden_size, hidden_size)`
        bias_ih: the learnable input-hidden bias, of shape `(3*hidden_size)`
        bias_hh: the learnable hidden-hidden bias, of shape `(3*hidden_size)`

    .. note::
        All the weights and biases are initialized from :math:`\mathcal{U}(-\sqrt{k}, \sqrt{k})`
        where :math:`k = \frac{1}{\text{hidden\_size}}`

    For example:

    .. code-block:: python
        >>> import oneflow as flow
        >>> import oneflow.nn as nn

        >>> rnn = nn.GRUCell(10, 20)
        >>> input = flow.randn(6, 3, 10)
        >>> hx = flow.randn(3, 20)
        >>> hx = rnn(input[0], hx)
        >>> hx.size()
        oneflow.Size([3, 20])
    """

    def __init__(
        self,
        input_size: int,
        hidden_size: int,
        bias: bool = True,
        device=None,
        dtype=None,
    ):
        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__(input_size, hidden_size, bias, num_chunks=3, **factory_kwargs)

    def forward(self, input: Tensor, hx: Optional[Tensor] = None) -> Tensor:
        assert input.dim() in (
            1,
            2,
        ), f"GRUCell: Expected input to be 1-D or 2-D but received {input.dim()}-D tensor"
        is_batched = input.dim() == 2
        if not is_batched:
            input = input.unsqueeze(0)

        if hx is None:
            if input.is_global():
                hx = flow.zeros(
                    input.size(0),
                    self.hidden_size,
                    dtype=input.dtype,
                    sbp=input.sbp,
                    placement=input.placement,
                )
            else:
                hx = flow.zeros(
                    input.size(0),
                    self.hidden_size,
                    dtype=input.dtype,
                    device=input.device,
                )
        else:
            hx = hx.unsqueeze(0) if not is_batched else hx

        ret = flow._C.gru_cell(
            input, hx, self.weight_ih, self.weight_hh, self.bias_ih, self.bias_hh,
        )

        if not is_batched:
            ret = ret.squeeze(0)

        return ret


if __name__ == "__main__":
    import doctest

    doctest.testmod(raise_on_error=True)
