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mmpretrain.models.heads.vision_transformer_head 源代码

# Copyright (c) OpenMMLab. All rights reserved.
import math
from collections import OrderedDict
from typing import List, Optional, Tuple

import torch
import torch.nn as nn
from mmcv.cnn import build_activation_layer
from mmengine.model import Sequential
from mmengine.model.weight_init import trunc_normal_

from mmpretrain.registry import MODELS
from .cls_head import ClsHead


[文档]@MODELS.register_module() class VisionTransformerClsHead(ClsHead): """Vision Transformer classifier head. Args: num_classes (int): Number of categories excluding the background category. in_channels (int): Number of channels in the input feature map. hidden_dim (int, optional): Number of the dimensions for hidden layer. Defaults to None, which means no extra hidden layer. act_cfg (dict): The activation config. Only available during pre-training. Defaults to ``dict(type='Tanh')``. init_cfg (dict): The extra initialization configs. Defaults to ``dict(type='Constant', layer='Linear', val=0)``. """ def __init__(self, num_classes: int, in_channels: int, hidden_dim: Optional[int] = None, act_cfg: dict = dict(type='Tanh'), init_cfg: dict = dict(type='Constant', layer='Linear', val=0), **kwargs): super(VisionTransformerClsHead, self).__init__( init_cfg=init_cfg, **kwargs) self.in_channels = in_channels self.num_classes = num_classes self.hidden_dim = hidden_dim self.act_cfg = act_cfg if self.num_classes <= 0: raise ValueError( f'num_classes={num_classes} must be a positive integer') self._init_layers() def _init_layers(self): """"Init hidden layer if exists.""" if self.hidden_dim is None: layers = [('head', nn.Linear(self.in_channels, self.num_classes))] else: layers = [ ('pre_logits', nn.Linear(self.in_channels, self.hidden_dim)), ('act', build_activation_layer(self.act_cfg)), ('head', nn.Linear(self.hidden_dim, self.num_classes)), ] self.layers = Sequential(OrderedDict(layers))
[文档] def init_weights(self): """"Init weights of hidden layer if exists.""" super(VisionTransformerClsHead, self).init_weights() # Modified from ClassyVision if hasattr(self.layers, 'pre_logits'): # Lecun norm trunc_normal_( self.layers.pre_logits.weight, std=math.sqrt(1 / self.layers.pre_logits.in_features)) nn.init.zeros_(self.layers.pre_logits.bias)
[文档] def pre_logits(self, feats: Tuple[List[torch.Tensor]]) -> torch.Tensor: """The process before the final classification head. The input ``feats`` is a tuple of list of tensor, and each tensor is the feature of a backbone stage. In ``VisionTransformerClsHead``, we obtain the feature of the last stage and forward in hidden layer if exists. """ feat = feats[-1] # Obtain feature of the last scale. # For backward-compatibility with the previous ViT output cls_token = feat[-1] if isinstance(feat, list) else feat if self.hidden_dim is None: return cls_token else: x = self.layers.pre_logits(cls_token) return self.layers.act(x)
[文档] def forward(self, feats: Tuple[List[torch.Tensor]]) -> torch.Tensor: """The forward process.""" pre_logits = self.pre_logits(feats) # The final classification head. cls_score = self.layers.head(pre_logits) return cls_score
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