layers.py

r"""
nn modules to replace Megatron's native ones
"""
import math
import numpy as np
import torch
import torch.nn as nn
import torch.nn.functional as F

from fmoe.transformer import FMoETransformerMLP
from .balance import reset_gate_hook
from .balance import generate_megatron_gate_hook


class _FakeMegatronMLP(nn.Module):
    r"""
    A fake mlp without model parallelism for correctness testing
    """

    def __init__(self, args, _):
        super().__init__()
        self.fc1 = nn.Linear(args.hidden_size, args.hidden_hidden_size)
        self.fc2 = nn.Linear(args.hidden_hidden_size, args.hidden_size)

    def forward(self, x):
        r"""
        Directly use GeLU
        """
        x = self.fc1(x)
        x = F.gelu(x)
        x = self.fc2(x)
        return x, torch.zeros_like(x)


def _megatron_init_method(self, rng, sigma):
    r"""
    Init method based on N(0, sigma).
    Copied from Megatron-LM
    """
    device = self.weight.device
    dtype = self.weight.dtype
    weight = rng.normal(loc=0.0, scale=sigma, size=tuple(self.weight.size()))
    self.weight.data = torch.from_numpy(weight).to(dtype=dtype, device=device)

    if self.bias is not None:
        # Always initialize bias to zero.
        with torch.no_grad():
            self.bias.zero_()


def _random_init_weight(self, rng):
    r"""
    Copied from torch.nn.init.kaiming_uniform_
    """
    fan = nn.init._calculate_correct_fan(self.weight[0], "fan_in")
    gain = nn.init.calculate_gain("leaky_relu", math.sqrt(5))
    std = gain / math.sqrt(fan)
    bound = math.sqrt(3.0) * std
    device = self.weight.device
    dtype = self.weight.dtype
    weight = rng.uniform(-bound, bound, size=tuple(self.weight.size()))
    self.weight.data = torch.from_numpy(weight).to(dtype=dtype, device=device)

    if self.bias is not None:
        fan_in, _ = nn.init._calculate_fan_in_and_fan_out(self.weight[0])
        bound = 1 / math.sqrt(fan_in)
        bias = rng.uniform(-bound, bound, size=tuple(self.bias.size()))
        self.bias.data = torch.from_numpy(bias).to(dtype=dtype, device=device)


class MegatronMLP(FMoETransformerMLP):
    r"""
    Make the FMoETransformerMLP layer that distributes experts across
    communication group `group` to replace the original MLP layer in Megatron.
    """

    def __init__(self, args, layer_idx, gate=None):
        if not args.distributed_experts:
            world_size = 1
            moe_group = None
        else:
            world_size = args.data_parallel_size
            from megatron.mpu import get_data_parallel_group
            moe_group = get_data_parallel_group()

        if not args.balance_strategy or args.balance_strategy == "naive":
            from fmoe.gates import NaiveGate
            gate = NaiveGate
        elif args.balance_strategy == "noisy":
            from fmoe.gates import NoisyGate
            gate = NoisyGate
        elif args.balance_strategy == "gshard":
            from fmoe.gates import GShardGate
            gate = GShardGate
        elif args.balance_strategy == "switch":
            from fmoe.gates import SwitchGate
            gate = SwitchGate
        elif args.balance_strategy == "swipe":
            from fmoe.gates import SwipeGate
            gate = SwipeGate
        elif gate is None:
            assert False, "Undefined balance strategy {}" % (args.balance_strategy)

        super().__init__(
            args.num_experts,
            top_k=args.top_k,
            d_model=args.hidden_size,
            d_hidden=args.hidden_hidden_size,
            world_size=world_size,
            moe_group=moe_group,
            expert_dp_comm="none" if args.distributed_experts else "dp",
            gate_hook=generate_megatron_gate_hook(
                layer_idx, args.num_experts * world_size
            ),
            gate=gate,
        )
        self.hidden_size = args.hidden_size
        if args.distributed_experts:
            self.rank = args.rank
        else:
            self.rank = 0
        self.sigma = args.init_method_std
        self.num_layers = args.num_layers
        self.reset_parameters()

    def reset_parameters(self):
        r"""
        Initialize the weight as linear layers.
        As megatron is using fixed random seed for some nasty stuff, an
        additional numpy rng is used.
        """
        rng = np.random.default_rng(np.random.randint(2048) + self.rank)
        
        if type(self.experts) is nn.ModuleList:
            for expert in self.experts:
                _megatron_init_method(expert.htoh4, rng, self.sigma)
        else:
            _megatron_init_method(self.experts.htoh4, rng, self.sigma)
        
        std = self.sigma / math.sqrt(2.0 * self.num_layers)
        
        if type(self.experts) is nn.ModuleList:
            for expert in self.experts:
                _megatron_init_method(expert.h4toh, rng, std)
        else:
            _megatron_init_method(self.experts.h4toh, rng, std)

    def forward(self, inp):
        from megatron import mpu
        x = super().forward(inp)
        x = mpu.reduce_from_tensor_model_parallel_region(x)
        return (
            x,
            torch.zeros(self.hidden_size, dtype=inp.dtype, device=inp.device),
        )


def fmoefy(
    model,
    num_experts=None,
    distributed_experts=True,
    hidden_hidden_size=None,
    top_k=None,
    gate=None,
    megatron_version=None
):
    r"""
    Replace MLP layers in a transformer-based model in Megatron by MoE.
    * `model` should be a standard Megatron model that has
    `model.language_model.transformer.layers` as transformer layers, which is an
    array of transformer blocks that contain an `mlp` member.
    * `distributed_expert` is set to True if different experts are located in
    different workers. Otherwise, the experts on the workers are identical, and
    they are trained in data-parallel mode. This can be useful when testing on
    small models that do not require high training throughput or large parameter
    capacity.
    """
    from megatron import get_args

    args = get_args()

    # Set distributed_experts to None to use default setting in args
    if distributed_experts is not None:
        args.distributed_experts = distributed_experts

    if num_experts is not None:
        args.num_experts = num_experts
    assert (
        "num_experts" in args
    ), "num_experts should be specified in arguments or fmoefy function"

    if top_k is not None:
        args.top_k = top_k
    elif not hasattr(args, "top_k"):
        args.top_k = 2

    args.hidden_hidden_size = hidden_hidden_size

    if megatron_version == "v2.2":

        for idx, l in enumerate(model.language_model.transformer.layers):
            l.mlp = MegatronMLP(args, idx, gate=gate)

        # initialize gate hook
        num_layers = len(model.language_model.transformer.layers)
    elif megatron_version == "v2.5":
        
        for idx, l in enumerate(model.language_model.encoder.layers):
            l.mlp = MegatronMLP(args, idx, gate=gate)
        if hasattr(model.language_model, "decoder"):
            for idx, l in enumerate(model.language_model.decoder.layers):
                l.mlp = MegatronMLP(args, idx, gate=gate)

        # initialize gate hook
        num_layers = len(model.language_model.encoder.layers)
        if hasattr(model.language_model, "decoder"):
            num_layers += len(model.language_model.decoder.layers)
    else:
        assert False, f"megatron_version {megatron_version} not known."

    reset_gate_hook(num_layers)

    return model