# Copyright (c) OpenMMLab. All rights reserved.
import io
import logging
import os
import os.path as osp
import pkgutil
import re
import time
import warnings
from collections import OrderedDict
from importlib import import_module
from tempfile import TemporaryDirectory
from typing import Callable, List, Optional, Sequence, Union

import torch
import torchvision
from torch.optim import Optimizer

import mmcv
from ..fileio import FileClient
from ..fileio import load as load_file
from ..parallel import is_module_wrapper
from ..utils import digit_version, load_url, mkdir_or_exist
from .dist_utils import get_dist_info

ENV_MMCV_HOME = 'MMCV_HOME'
ENV_XDG_CACHE_HOME = 'XDG_CACHE_HOME'
DEFAULT_CACHE_DIR = '~/.cache'


def _get_mmcv_home():
    mmcv_home = os.path.expanduser(
        os.getenv(
            ENV_MMCV_HOME,
            os.path.join(
                os.getenv(ENV_XDG_CACHE_HOME, DEFAULT_CACHE_DIR), 'mmcv')))

    mkdir_or_exist(mmcv_home)
    return mmcv_home


def load_state_dict(module: torch.nn.Module,
                    state_dict: Union[dict, OrderedDict],
                    strict: bool = False,
                    logger: Optional[logging.Logger] = None) -> None:
    """Load state_dict to a module.

    This method is modified from :meth:`torch.nn.Module.load_state_dict`.
    Default value for ``strict`` is set to ``False`` and the message for
    param mismatch will be shown even if strict is False.

    Args:
        module (Module): Module that receives the state_dict.
        state_dict (OrderedDict): Weights.
        strict (bool): whether to strictly enforce that the keys
            in :attr:`state_dict` match the keys returned by this module's
            :meth:`~torch.nn.Module.state_dict` function. Default: ``False``.
        logger (:obj:`logging.Logger`, optional): Logger to log the error
            message. If not specified, print function will be used.
    """
    unexpected_keys: List = []
    all_missing_keys: List = []
    err_msg: List = []

    metadata = getattr(state_dict, '_metadata', None)
    state_dict = state_dict.copy()
    if metadata is not None:
        state_dict._metadata = metadata  # type: ignore

    # use _load_from_state_dict to enable checkpoint version control
    def load(module, prefix=''):
        # recursively check parallel module in case that the model has a
        # complicated structure, e.g., nn.Module(nn.Module(DDP))
        if is_module_wrapper(module):
            module = module.module
        local_metadata = {} if metadata is None else metadata.get(
            prefix[:-1], {})
        module._load_from_state_dict(state_dict, prefix, local_metadata, True,
                                     all_missing_keys, unexpected_keys,
                                     err_msg)
        for name, child in module._modules.items():
            if child is not None:
                load(child, prefix + name + '.')

    load(module)
    # break load->load reference cycle
    load = None  # type: ignore

    # ignore "num_batches_tracked" of BN layers
    missing_keys = [
        key for key in all_missing_keys if 'num_batches_tracked' not in key
    ]

    if unexpected_keys:
        err_msg.append('unexpected key in source '
                       f'state_dict: {", ".join(unexpected_keys)}\n')
    if missing_keys:
        err_msg.append(
            f'missing keys in source state_dict: {", ".join(missing_keys)}\n')

    rank, _ = get_dist_info()
    if len(err_msg) > 0 and rank == 0:
        err_msg.insert(
            0, 'The model and loaded state dict do not match exactly\n')
        err_msg = '\n'.join(err_msg)  # type: ignore
        if strict:
            raise RuntimeError(err_msg)
        elif logger is not None:
            logger.warning(err_msg)
        else:
            print(err_msg)


def get_torchvision_models():
    if digit_version(torchvision.__version__) < digit_version('0.13.0a0'):
        model_urls = dict()
        # When the version of torchvision is lower than 0.13, the model url is
        # not declared in `torchvision.model.__init__.py`, so we need to
        # iterate through `torchvision.models.__path__` to get the url for each
        # model.
        for _, name, ispkg in pkgutil.walk_packages(
                torchvision.models.__path__):
            if ispkg:
                continue
            _zoo = import_module(f'torchvision.models.{name}')
            if hasattr(_zoo, 'model_urls'):
                _urls = getattr(_zoo, 'model_urls')
                model_urls.update(_urls)
    else:
        # Since torchvision bumps to v0.13, the weight loading logic,
        # model keys and model urls have been changed. Here the URLs of old
        # version is loaded to avoid breaking back compatibility. If the
        # torchvision version>=0.13.0, new URLs will be added. Users can get
        # the resnet50 checkpoint by setting 'resnet50.imagent1k_v1',
        # 'resnet50' or 'ResNet50_Weights.IMAGENET1K_V1' in the config.
        json_path = osp.join(mmcv.__path__[0],
                             'model_zoo/torchvision_0.12.json')
        model_urls = mmcv.load(json_path)
        for cls_name, cls in torchvision.models.__dict__.items():
            # The name of torchvision model weights classes ends with
            # `_Weights` such as `ResNet18_Weights`. However, some model weight
            # classes, such as `MNASNet0_75_Weights` does not have any urls in
            # torchvision 0.13.0 and cannot be iterated. Here we simply check
            # `DEFAULT` attribute to ensure the class is not empty.
            if (not cls_name.endswith('_Weights')
                    or not hasattr(cls, 'DEFAULT')):
                continue
            # Since `cls.DEFAULT` can not be accessed by iterating cls, we set
            # default urls explicitly.
            cls_key = cls_name.replace('_Weights', '').lower()
            model_urls[f'{cls_key}.default'] = cls.DEFAULT.url
            for weight_enum in cls:
                cls_key = cls_name.replace('_Weights', '').lower()
                cls_key = f'{cls_key}.{weight_enum.name.lower()}'
                model_urls[cls_key] = weight_enum.url

    return model_urls


def get_external_models():
    mmcv_home = _get_mmcv_home()
    default_json_path = osp.join(mmcv.__path__[0], 'model_zoo/open_mmlab.json')
    default_urls = load_file(default_json_path)
    assert isinstance(default_urls, dict)
    external_json_path = osp.join(mmcv_home, 'open_mmlab.json')
    if osp.exists(external_json_path):
        external_urls = load_file(external_json_path)
        assert isinstance(external_urls, dict)
        default_urls.update(external_urls)

    return default_urls


def get_mmcls_models():
    mmcls_json_path = osp.join(mmcv.__path__[0], 'model_zoo/mmcls.json')
    mmcls_urls = load_file(mmcls_json_path)

    return mmcls_urls


def get_deprecated_model_names():
    deprecate_json_path = osp.join(mmcv.__path__[0],
                                   'model_zoo/deprecated.json')
    deprecate_urls = load_file(deprecate_json_path)
    assert isinstance(deprecate_urls, dict)

    return deprecate_urls


def _process_mmcls_checkpoint(checkpoint):
    if 'state_dict' in checkpoint:
        state_dict = checkpoint['state_dict']
    else:
        # Some checkpoints converted from 3rd-party repo don't
        # have the "state_dict" key.
        state_dict = checkpoint
    new_state_dict = OrderedDict()
    for k, v in state_dict.items():
        if k.startswith('backbone.'):
            new_state_dict[k[9:]] = v
    new_checkpoint = dict(state_dict=new_state_dict)

    return new_checkpoint


class CheckpointLoader:
    """A general checkpoint loader to manage all schemes."""

    _schemes: dict = {}

    @classmethod
    def _register_scheme(cls, prefixes, loader, force=False):
        if isinstance(prefixes, str):
            prefixes = [prefixes]
        else:
            assert isinstance(prefixes, (list, tuple))
        for prefix in prefixes:
            if (prefix not in cls._schemes) or force:
                cls._schemes[prefix] = loader
            else:
                raise KeyError(
                    f'{prefix} is already registered as a loader backend, '
                    'add "force=True" if you want to override it')
        # sort, longer prefixes take priority
        cls._schemes = OrderedDict(
            sorted(cls._schemes.items(), key=lambda t: t[0], reverse=True))

    @classmethod
    def register_scheme(cls,
                        prefixes: Union[str, Sequence[str]],
                        loader: Optional[Callable] = None,
                        force: bool = False):
        """Register a loader to CheckpointLoader.

        This method can be used as a normal class method or a decorator.

        Args:
            prefixes (str or Sequence[str]):
            The prefix of the registered loader.
            loader (function, optional): The loader function to be registered.
                When this method is used as a decorator, loader is None.
                Defaults to None.
            force (bool, optional): Whether to override the loader
                if the prefix has already been registered. Defaults to False.
        """

        if loader is not None:
            cls._register_scheme(prefixes, loader, force=force)
            return

        def _register(loader_cls):
            cls._register_scheme(prefixes, loader_cls, force=force)
            return loader_cls

        return _register

    @classmethod
    def _get_checkpoint_loader(cls, path):
        """Finds a loader that supports the given path. Falls back to the local
        loader if no other loader is found.

        Args:
            path (str): checkpoint path

        Returns:
            callable: checkpoint loader
        """
        for p in cls._schemes:
            # use regular match to handle some cases that where the prefix of
            # loader has a prefix. For example, both 's3://path' and
            # 'open-mmlab:s3://path' should return `load_from_ceph`
            if re.match(p, path) is not None:
                return cls._schemes[p]

    @classmethod
    def load_checkpoint(
            cls,
            filename: str,
            map_location: Union[str, Callable, None] = None,
            logger: Optional[logging.Logger] = None
    ) -> Union[dict, OrderedDict]:
        """load checkpoint through URL scheme path.

        Args:
            filename (str): checkpoint file name with given prefix
            map_location (str, optional): Same as :func:`torch.load`.
                Default: None
            logger (:mod:`logging.Logger`, optional): The logger for message.
                Default: None

        Returns:
            dict or OrderedDict: The loaded checkpoint.
        """

        checkpoint_loader = cls._get_checkpoint_loader(filename)
        class_name = checkpoint_loader.__name__
        mmcv.print_log(
            f'load checkpoint from {class_name[10:]} path: {filename}', logger)
        return checkpoint_loader(filename, map_location)


@CheckpointLoader.register_scheme(prefixes='')
def load_from_local(
    filename: str,
    map_location: Union[str, Callable, None] = None,
) -> Union[dict, OrderedDict]:
    """load checkpoint by local file path.

    Args:
        filename (str): local checkpoint file path
        map_location (str, optional): Same as :func:`torch.load`.

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """
    filename = osp.expanduser(filename)
    if not osp.isfile(filename):
        raise FileNotFoundError(f'{filename} can not be found.')
    checkpoint = torch.load(filename, map_location=map_location)
    return checkpoint


@CheckpointLoader.register_scheme(prefixes=('http://', 'https://'))
def load_from_http(
        filename: str,
        map_location: Union[str, Callable, None] = None,
        model_dir: Optional[str] = None) -> Union[dict, OrderedDict]:
    """load checkpoint through HTTP or HTTPS scheme path. In distributed
    setting, this function only download checkpoint at local rank 0.

    Args:
        filename (str): checkpoint file path with modelzoo or
            torchvision prefix
        map_location (str, optional): Same as :func:`torch.load`.
        model_dir (str, optional): directory in which to save the object,
            Default: None

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """
    rank, world_size = get_dist_info()
    if rank == 0:
        checkpoint = load_url(
            filename, model_dir=model_dir, map_location=map_location)
    if world_size > 1:
        torch.distributed.barrier()
        if rank > 0:
            checkpoint = load_url(
                filename, model_dir=model_dir, map_location=map_location)
    return checkpoint


@CheckpointLoader.register_scheme(prefixes='pavi://')
def load_from_pavi(
    filename: str,
    map_location: Union[str, Callable, None] = None,
) -> Union[dict, OrderedDict]:
    """load checkpoint through the file path prefixed with pavi. In distributed
    setting, this function download ckpt at all ranks to different temporary
    directories.

    Args:
        filename (str): checkpoint file path with pavi prefix
        map_location (str, optional): Same as :func:`torch.load`.
          Default: None

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """
    assert filename.startswith('pavi://'), \
        f'Expected filename startswith `pavi://`, but get {filename}'
    model_path = filename[7:]

    try:
        from pavi import modelcloud
    except ImportError:
        raise ImportError(
            'Please install pavi to load checkpoint from modelcloud.')

    model = modelcloud.get(model_path)
    with TemporaryDirectory() as tmp_dir:
        downloaded_file = osp.join(tmp_dir, model.name)
        model.download(downloaded_file)
        checkpoint = torch.load(downloaded_file, map_location=map_location)
    return checkpoint


@CheckpointLoader.register_scheme(prefixes=r'(\S+\:)?s3://')
def load_from_ceph(filename: str,
                   map_location: Union[str, Callable, None] = None,
                   backend: str = 'petrel') -> Union[dict, OrderedDict]:
    """load checkpoint through the file path prefixed with s3.  In distributed
    setting, this function download ckpt at all ranks to different temporary
    directories.

    Note:
        Since v1.4.1, the registered scheme prefixes have been enhanced to
        support bucket names in the path prefix, e.g. 's3://xx.xx/xx.path',
        'bucket1:s3://xx.xx/xx.path'.

    Args:
        filename (str): checkpoint file path with s3 prefix
        map_location (str, optional): Same as :func:`torch.load`.
        backend (str): The storage backend type. Options are 'ceph',
            'petrel'. Default: 'petrel'.

    .. warning::
        :class:`mmcv.fileio.file_client.CephBackend` will be deprecated,
        please use :class:`mmcv.fileio.file_client.PetrelBackend` instead.

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """
    allowed_backends = ['ceph', 'petrel']
    if backend not in allowed_backends:
        raise ValueError(f'Load from Backend {backend} is not supported.')

    if backend == 'ceph':
        warnings.warn(
            'CephBackend will be deprecated, please use PetrelBackend instead',
            DeprecationWarning)

    # CephClient and PetrelBackend have the same prefix 's3://' and the latter
    # will be chosen as default. If PetrelBackend can not be instantiated
    # successfully, the CephClient will be chosen.
    try:
        file_client = FileClient(backend=backend)
    except ImportError:
        allowed_backends.remove(backend)
        file_client = FileClient(backend=allowed_backends[0])

    with io.BytesIO(file_client.get(filename)) as buffer:
        checkpoint = torch.load(buffer, map_location=map_location)
    return checkpoint


@CheckpointLoader.register_scheme(prefixes=('modelzoo://', 'torchvision://'))
def load_from_torchvision(
    filename: str,
    map_location: Union[str, Callable, None] = None,
) -> Union[dict, OrderedDict]:
    """load checkpoint through the file path prefixed with modelzoo or
    torchvision.

    Args:
        filename (str): checkpoint file path with modelzoo or
            torchvision prefix
        map_location (str, optional): Same as :func:`torch.load`.

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """
    model_urls = get_torchvision_models()
    if filename.startswith('modelzoo://'):
        warnings.warn(
            'The URL scheme of "modelzoo://" is deprecated, please '
            'use "torchvision://" instead', DeprecationWarning)
        model_name = filename[11:]
    else:
        model_name = filename[14:]

    # Support getting model urls in the same way as torchvision
    # `ResNet50_Weights.IMAGENET1K_V1` will be mapped to
    # resnet50.imagenet1k_v1.
    model_name = model_name.lower().replace('_weights', '')
    return load_from_http(model_urls[model_name], map_location=map_location)


@CheckpointLoader.register_scheme(prefixes=('open-mmlab://', 'openmmlab://'))
def load_from_openmmlab(
    filename: str,
    map_location: Union[str, Callable, None] = None,
) -> Union[dict, OrderedDict]:
    """load checkpoint through the file path prefixed with open-mmlab or
    openmmlab.

    Args:
        filename (str): checkpoint file path with open-mmlab or
        openmmlab prefix
        map_location (str, optional): Same as :func:`torch.load`.
          Default: None

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """

    model_urls = get_external_models()
    prefix_str = 'open-mmlab://'
    if filename.startswith(prefix_str):
        model_name = filename[13:]
    else:
        model_name = filename[12:]
        prefix_str = 'openmmlab://'

    deprecated_urls = get_deprecated_model_names()
    if model_name in deprecated_urls:
        warnings.warn(
            f'{prefix_str}{model_name} is deprecated in favor '
            f'of {prefix_str}{deprecated_urls[model_name]}',
            DeprecationWarning)
        model_name = deprecated_urls[model_name]
    model_url = model_urls[model_name]
    # check if is url
    if model_url.startswith(('http://', 'https://')):
        checkpoint = load_from_http(model_url, map_location=map_location)
    else:
        filename = osp.join(_get_mmcv_home(), model_url)
        if not osp.isfile(filename):
            raise FileNotFoundError(f'{filename} can not be found.')
        checkpoint = torch.load(filename, map_location=map_location)
    return checkpoint


@CheckpointLoader.register_scheme(prefixes='mmcls://')
def load_from_mmcls(
    filename: str,
    map_location: Union[str, Callable, None] = None,
) -> Union[dict, OrderedDict]:
    """load checkpoint through the file path prefixed with mmcls.

    Args:
        filename (str): checkpoint file path with mmcls prefix
        map_location (str, optional): Same as :func:`torch.load`.

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """

    model_urls = get_mmcls_models()
    model_name = filename[8:]
    checkpoint = load_from_http(
        model_urls[model_name], map_location=map_location)
    checkpoint = _process_mmcls_checkpoint(checkpoint)
    return checkpoint


def _load_checkpoint(
        filename: str,
        map_location: Union[str, Callable, None] = None,
        logger: Optional[logging.Logger] = None) -> Union[dict, OrderedDict]:
    """Load checkpoint from somewhere (modelzoo, file, url).

    Args:
        filename (str): Accept local filepath, URL, ``torchvision://xxx``,
            ``open-mmlab://xxx``. Please refer to ``docs/model_zoo.md`` for
            details.
        map_location (str, optional): Same as :func:`torch.load`.
           Default: None.
        logger (:mod:`logging.Logger`, optional): The logger for error message.
           Default: None

    Returns:
        dict or OrderedDict: The loaded checkpoint. It can be either an
           OrderedDict storing model weights or a dict containing other
           information, which depends on the checkpoint.
    """
    return CheckpointLoader.load_checkpoint(filename, map_location, logger)


def _load_checkpoint_with_prefix(
    prefix: str,
    filename: str,
    map_location: Union[str, Callable, None] = None,
) -> Union[dict, OrderedDict]:
    """Load partial pretrained model with specific prefix.

    Args:
        prefix (str): The prefix of sub-module.
        filename (str): Accept local filepath, URL, ``torchvision://xxx``,
            ``open-mmlab://xxx``. Please refer to ``docs/model_zoo.md`` for
            details.
        map_location (str | None): Same as :func:`torch.load`. Default: None.

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """

    checkpoint = _load_checkpoint(filename, map_location=map_location)

    if 'state_dict' in checkpoint:
        state_dict = checkpoint['state_dict']
    else:
        state_dict = checkpoint
    if not prefix.endswith('.'):
        prefix += '.'
    prefix_len = len(prefix)

    state_dict = {
        k[prefix_len:]: v
        for k, v in state_dict.items() if k.startswith(prefix)
    }

    assert state_dict, f'{prefix} is not in the pretrained model'
    return state_dict


def load_checkpoint(
        model: torch.nn.Module,
        filename: str,
        map_location: Union[str, Callable, None] = None,
        strict: bool = False,
        logger: Optional[logging.Logger] = None,
        revise_keys: list = [(r'^module\.', '')]) -> Union[dict, OrderedDict]:
    """Load checkpoint from a file or URI.

    Args:
        model (Module): Module to load checkpoint.
        filename (str): Accept local filepath, URL, ``torchvision://xxx``,
            ``open-mmlab://xxx``. Please refer to ``docs/model_zoo.md`` for
            details.
        map_location (str): Same as :func:`torch.load`.
        strict (bool): Whether to allow different params for the model and
            checkpoint.
        logger (:mod:`logging.Logger` or None): The logger for error message.
        revise_keys (list): A list of customized keywords to modify the
            state_dict in checkpoint. Each item is a (pattern, replacement)
            pair of the regular expression operations. Default: strip
            the prefix 'module.' by [(r'^module\\.', '')].

    Returns:
        dict or OrderedDict: The loaded checkpoint.
    """
    checkpoint = _load_checkpoint(filename, map_location, logger)
    # OrderedDict is a subclass of dict
    if not isinstance(checkpoint, dict):
        raise RuntimeError(
            f'No state_dict found in checkpoint file {filename}')
    # get state_dict from checkpoint
    if 'state_dict' in checkpoint:
        state_dict = checkpoint['state_dict']
    else:
        state_dict = checkpoint

    # strip prefix of state_dict
    metadata = getattr(state_dict, '_metadata', OrderedDict())
    for p, r in revise_keys:
        state_dict = OrderedDict(
            {re.sub(p, r, k): v
             for k, v in state_dict.items()})
    # Keep metadata in state_dict
    state_dict._metadata = metadata

    # load state_dict
    load_state_dict(model, state_dict, strict, logger)
    return checkpoint


def weights_to_cpu(state_dict: OrderedDict) -> OrderedDict:
    """Copy a model state_dict to cpu.

    Args:
        state_dict (OrderedDict): Model weights on GPU.

    Returns:
        OrderedDict: Model weights on GPU.
    """
    state_dict_cpu = OrderedDict()
    for key, val in state_dict.items():
        state_dict_cpu[key] = val.cpu()
    # Keep metadata in state_dict
    state_dict_cpu._metadata = getattr(  # type: ignore
        state_dict, '_metadata', OrderedDict())
    return state_dict_cpu


def _save_to_state_dict(module: torch.nn.Module, destination: dict,
                        prefix: str, keep_vars: bool) -> None:
    """Saves module state to `destination` dictionary.

    This method is modified from :meth:`torch.nn.Module._save_to_state_dict`.

    Args:
        module (nn.Module): The module to generate state_dict.
        destination (dict): A dict where state will be stored.
        prefix (str): The prefix for parameters and buffers used in this
            module.
    """
    for name, param in module._parameters.items():
        if param is not None:
            destination[prefix + name] = param if keep_vars else param.detach()
    for name, buf in module._buffers.items():
        # remove check of _non_persistent_buffers_set to allow nn.BatchNorm2d
        if buf is not None:
            destination[prefix + name] = buf if keep_vars else buf.detach()


def get_state_dict(module: torch.nn.Module,
                   destination: Optional[OrderedDict] = None,
                   prefix: str = '',
                   keep_vars: bool = False) -> OrderedDict:
    """Returns a dictionary containing a whole state of the module.

    Both parameters and persistent buffers (e.g. running averages) are
    included. Keys are corresponding parameter and buffer names.

    This method is modified from :meth:`torch.nn.Module.state_dict` to
    recursively check parallel module in case that the model has a complicated
    structure, e.g., nn.Module(nn.Module(DDP)).

    Args:
        module (nn.Module): The module to generate state_dict.
        destination (OrderedDict): Returned dict for the state of the
            module.
        prefix (str): Prefix of the key.
        keep_vars (bool): Whether to keep the variable property of the
            parameters. Default: False.

    Returns:
        dict: A dictionary containing a whole state of the module.
    """
    # recursively check parallel module in case that the model has a
    # complicated structure, e.g., nn.Module(nn.Module(DDP))
    if is_module_wrapper(module):
        module = module.module

    # below is the same as torch.nn.Module.state_dict()
    if destination is None:
        destination = OrderedDict()
        destination._metadata = OrderedDict()  # type: ignore
    destination._metadata[prefix[:-1]] = local_metadata = dict(  # type: ignore
        version=module._version)
    _save_to_state_dict(module, destination, prefix, keep_vars)
    for name, child in module._modules.items():
        if child is not None:
            get_state_dict(
                child, destination, prefix + name + '.', keep_vars=keep_vars)
    for hook in module._state_dict_hooks.values():
        hook_result = hook(module, destination, prefix, local_metadata)
        if hook_result is not None:
            destination = hook_result
    return destination  # type: ignore


def save_checkpoint(model: torch.nn.Module,
                    filename: str,
                    optimizer: Optional[Optimizer] = None,
                    meta: Optional[dict] = None,
                    file_client_args: Optional[dict] = None) -> None:
    """Save checkpoint to file.

    The checkpoint will have 3 fields: ``meta``, ``state_dict`` and
    ``optimizer``. By default ``meta`` will contain version and time info.

    Args:
        model (Module): Module whose params are to be saved.
        filename (str): Checkpoint filename.
        optimizer (:obj:`Optimizer`, optional): Optimizer to be saved.
        meta (dict, optional): Metadata to be saved in checkpoint.
        file_client_args (dict, optional): Arguments to instantiate a
            FileClient. See :class:`mmcv.fileio.FileClient` for details.
            Default: None.
            `New in version 1.3.16.`
    """
    if meta is None:
        meta = {}
    elif not isinstance(meta, dict):
        raise TypeError(f'meta must be a dict or None, but got {type(meta)}')
    meta.update(mmcv_version=mmcv.__version__, time=time.asctime())

    if is_module_wrapper(model):
        model = model.module

    if hasattr(model, 'CLASSES') and model.CLASSES is not None:
        # save class name to the meta
        meta.update(CLASSES=model.CLASSES)

    checkpoint = {
        'meta': meta,
        'state_dict': weights_to_cpu(get_state_dict(model))
    }
    # save optimizer state dict in the checkpoint
    if isinstance(optimizer, Optimizer):
        checkpoint['optimizer'] = optimizer.state_dict()
    elif isinstance(optimizer, dict):
        checkpoint['optimizer'] = {}
        for name, optim in optimizer.items():
            checkpoint['optimizer'][name] = optim.state_dict()

    if filename.startswith('pavi://'):
        if file_client_args is not None:
            raise ValueError(
                'file_client_args should be "None" if filename starts with'
                f'"pavi://", but got {file_client_args}')
        try:
            from pavi import exception, modelcloud
        except ImportError:
            raise ImportError(
                'Please install pavi to load checkpoint from modelcloud.')
        model_path = filename[7:]
        root = modelcloud.Folder()
        model_dir, model_name = osp.split(model_path)
        try:
            model = modelcloud.get(model_dir)
        except exception.NodeNotFoundError:
            model = root.create_training_model(model_dir)
        with TemporaryDirectory() as tmp_dir:
            checkpoint_file = osp.join(tmp_dir, model_name)
            with open(checkpoint_file, 'wb') as f:
                torch.save(checkpoint, f)
                f.flush()
            model.create_file(checkpoint_file, name=model_name)
    else:
        file_client = FileClient.infer_client(file_client_args, filename)
        with io.BytesIO() as f:
            torch.save(checkpoint, f)
            file_client.put(f.getvalue(), filename)
