mmcls/models/necks/reduction.py

# Copyright (c) OpenMMLab. All rights reserved.
import copy
from typing import Optional, Tuple, Union

import torch
import torch.nn as nn
from mmcv.cnn import build_activation_layer, build_norm_layer
from mmengine.model import BaseModule

from mmcls.registry import MODELS


@MODELS.register_module()
class LinearReduction(BaseModule):
    """Neck with Dimension reduction.

    Args:
        in_channels (int): Number of channels in the input.
        out_channels (int): Number of channels in the output.
        norm_cfg (dict, optional): dictionary to construct and
            config norm layer. Defaults to dict(type='BN1d').
        act_cfg (dict, optional): dictionary to construct and
            config activate layer. Defaults to None.
        init_cfg (dict, optional): dictionary to initialize weights.
            Defaults to None.
    """

    def __init__(self,
                 in_channels: int,
                 out_channels: int,
                 norm_cfg: Optional[dict] = dict(type='BN1d'),
                 act_cfg: Optional[dict] = None,
                 init_cfg: Optional[dict] = None):
        super(LinearReduction, self).__init__(init_cfg=init_cfg)

        self.in_channels = in_channels
        self.out_channels = out_channels
        self.norm_cfg = copy.deepcopy(norm_cfg)
        self.act_cfg = copy.deepcopy(act_cfg)

        self.reduction = nn.Linear(
            in_features=in_channels, out_features=out_channels)
        if norm_cfg:
            self.norm = build_norm_layer(norm_cfg, out_channels)[1]
        else:
            self.norm = nn.Identity()
        if act_cfg:
            self.act = build_activation_layer(act_cfg)
        else:
            self.act = nn.Identity()

    def forward(self, inputs: Union[Tuple,
                                    torch.Tensor]) -> Tuple[torch.Tensor]:
        """forward function.

        Args:
            inputs (Union[Tuple, torch.Tensor]): The features extracted from
                 the backbone. Multiple stage inputs are acceptable but only
                  the last stage will be used.

        Returns:
            Tuple(torch.Tensor)): A tuple of reducted features.
        """
        assert isinstance(inputs, (tuple, torch.Tensor)), (
            'The inputs of `LinearReduction` neck  must be tuple or '
            f'`torch.Tensor`, but get {type(inputs)}.')
        if isinstance(inputs, tuple):
            inputs = inputs[-1]

        out = self.act(self.norm(self.reduction(inputs)))
        return (out, )