PoolFormer.py

# (ref) https://github.com/sail-sg/poolformer

import copy
import torch
import torch.nn as nn

from timm.data import IMAGENET_DEFAULT_MEAN, IMAGENET_DEFAULT_STD
from timm.models.layers import DropPath, trunc_normal_
from timm.models.registry import register_model
from timm.models.layers.helpers import to_2tuple


def _cfg(url='', **kwargs):
    return {
        'url': url,
        'num_classes': 1000, 'input_size': (3, 224, 224), 'pool_size': None,
        'crop_pct': .95, 'interpolation': 'bicubic',
        'mean': IMAGENET_DEFAULT_MEAN, 'std': IMAGENET_DEFAULT_STD, 
        'classifier': 'head',
        **kwargs
    }


default_cfgs = {
    'poolformer_s': _cfg(crop_pct=0.9),
    'poolformer_m': _cfg(crop_pct=0.95),
}


class PatchEmbed(nn.Module):
    """
    Patch Embedding (Stem) that is implemented by a layer of conv. 
    Input: tensor in shape [B, C, H, W]
    Output: tensor in shape [B, C, H/stride, W/stride]
    """
    def __init__(self, patch_size=16, stride=16, padding=0, 
                in_chans=3, embed_dim=768, norm_layer=None):
        super().__init__()
        patch_size = to_2tuple(patch_size)
        stride = to_2tuple(stride)
        padding = to_2tuple(padding)
        self.proj = nn.Conv2d(in_chans, embed_dim, kernel_size=patch_size, 
                            stride=stride, padding=padding)
        self.norm = norm_layer(embed_dim) if norm_layer else nn.Identity()

    def forward(self, x):
        x = self.proj(x)
        x = self.norm(x)
        return x

# ------- 

class LayerNormChannel(nn.Module):
    """
    LayerNorm only for Channel Dimension.
    Input: tensor in shape [B, C, H, W]
    """
    def __init__(self, num_channels, eps=1e-05):
        super().__init__()
        self.weight = nn.Parameter(torch.ones(num_channels))
        self.bias = nn.Parameter(torch.zeros(num_channels))
        self.eps = eps

    def forward(self, x):
        u = x.mean(1, keepdim=True)
        s = (x - u).pow(2).mean(1, keepdim=True)
        x = (x - u) / torch.sqrt(s + self.eps)
        x = self.weight.unsqueeze(-1).unsqueeze(-1) * x \
            + self.bias.unsqueeze(-1).unsqueeze(-1)
        return x



class GroupNorm(nn.GroupNorm):
    """
    Group Normalization with 1 group.
    Input: tensor in shape [B, C, H, W]
    """
    def __init__(self, num_channels, **kwargs):
        super().__init__(1, num_channels, **kwargs)

# ------- 

class Pooling(nn.Module):
    """
    Implementation of pooling for PoolFormer
    --pool_size: pooling size
    """
    def __init__(self, pool_size=3):
        super().__init__()
        self.pool = nn.AvgPool2d(
            pool_size, stride=1, padding=pool_size//2, count_include_pad=False)

    def forward(self, x):
        return self.pool(x) - x


class Mlp(nn.Module):
    """
    Implementation of MLP with 1*1 convolutions.
    Input: tensor with shape [B, C, H, W]
    """
    def __init__(self, in_features, hidden_features=None, 
                out_features=None, act_layer=nn.GELU, drop=0.):
        super().__init__()
        out_features = out_features or in_features
        hidden_features = hidden_features or in_features
        self.fc1 = nn.Conv2d(in_features, hidden_features, 1)
        self.act = act_layer()
        self.fc2 = nn.Conv2d(hidden_features, out_features, 1)
        self.drop = nn.Dropout(drop)
        self.apply(self._init_weights)

    def _init_weights(self, m):
        if isinstance(m, nn.Conv2d):
            trunc_normal_(m.weight, std=.02)
            if m.bias is not None:
                nn.init.constant_(m.bias, 0)

    def forward(self, x):
        x = self.fc1(x)
        x = self.act(x)
        x = self.drop(x)
        x = self.fc2(x)
        x = self.drop(x)
        return x

# ------- 

class PoolFormerBlock(nn.Module):
    """
    Implementation of one PoolFormer block.
    --dim: embedding dim
    --pool_size: pooling size
    --mlp_ratio: mlp expansion ratio
    --act_layer: activation
    --norm_layer: normalization
    --drop: dropout rate
    --drop path: Stochastic Depth, 
        refer to https://arxiv.org/abs/1603.09382
    --use_layer_scale, --layer_scale_init_value: LayerScale, 
        refer to https://arxiv.org/abs/2103.17239
    """
    def __init__(self, dim, pool_size=3, mlp_ratio=4., 
                 act_layer=nn.GELU, norm_layer=GroupNorm, 
                 drop=0., drop_path=0., 
                 use_layer_scale=True, layer_scale_init_value=1e-5):

        super().__init__()

        self.norm1 = norm_layer(dim)
        self.token_mixer = Pooling(pool_size=pool_size)
        self.norm2 = norm_layer(dim)
        mlp_hidden_dim = int(dim * mlp_ratio)
        self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, 
                       act_layer=act_layer, drop=drop)

        # The following two techniques are useful to train deep PoolFormers.
        self.drop_path = DropPath(drop_path) if drop_path > 0. \
            else nn.Identity()
        self.use_layer_scale = use_layer_scale
        if use_layer_scale:
            self.layer_scale_1 = nn.Parameter(
                layer_scale_init_value * torch.ones((dim)), requires_grad=True)
            self.layer_scale_2 = nn.Parameter(
                layer_scale_init_value * torch.ones((dim)), requires_grad=True)

    def forward(self, x):
        if self.use_layer_scale:
            x = x + self.drop_path(
                self.layer_scale_1.unsqueeze(-1).unsqueeze(-1)
                * self.token_mixer(self.norm1(x)))
            x = x + self.drop_path(
                self.layer_scale_2.unsqueeze(-1).unsqueeze(-1)
                * self.mlp(self.norm2(x)))
        else:
            x = x + self.drop_path(self.token_mixer(self.norm1(x)))
            x = x + self.drop_path(self.mlp(self.norm2(x)))
        return x


def basic_blocks(dim, index, layers, 
                 pool_size=3, mlp_ratio=4., 
                 act_layer=nn.GELU, norm_layer=GroupNorm, 
                 drop_rate=.0, drop_path_rate=0., 
                 use_layer_scale=True, layer_scale_init_value=1e-5):
    """
    generate PoolFormer blocks for a stage
    return: PoolFormer blocks 
    """
    blocks = []
    for block_idx in range(layers[index]):
        block_dpr = drop_path_rate * (
            block_idx + sum(layers[:index])) / (sum(layers) - 1)
        blocks.append(PoolFormerBlock(
            dim, pool_size=pool_size, mlp_ratio=mlp_ratio, 
            act_layer=act_layer, norm_layer=norm_layer, 
            drop=drop_rate, drop_path=block_dpr, 
            use_layer_scale=use_layer_scale, 
            layer_scale_init_value=layer_scale_init_value, 
            ))
    blocks = nn.Sequential(*blocks)

    return blocks        

# ------- 

class PoolFormer(nn.Module):
    """
    PoolFormer, the main class of our model
    --layers: [x,x,x,x], number of blocks for the 4 stages
    --embed_dims, --mlp_ratios, --pool_size: the embedding dims, mlp ratios and 
        pooling size for the 4 stages
    --downsamples: flags to apply downsampling or not
    --norm_layer, --act_layer: define the types of normalization and activation
    --num_classes: number of classes for the image classification
    --in_patch_size, --in_stride, --in_pad: specify the patch embedding
        for the input image
    --down_patch_size --down_stride --down_pad: 
        specify the downsample (patch embed.)
    --fork_feat: whether output features of the 4 stages, for dense prediction
    --init_cfg, --pretrained: 
        for mmdetection and mmsegmentation to load pretrained weights
    """
    def __init__(self, layers, embed_dims=None, 
                 mlp_ratios=None, downsamples=None, 
                 pool_size=3, 
                 norm_layer=GroupNorm, act_layer=nn.GELU, 
                 num_classes=1000,
                 in_patch_size=7, in_stride=4, in_pad=2, 
                 down_patch_size=3, down_stride=2, down_pad=1, 
                 drop_rate=0., drop_path_rate=0.,
                 use_layer_scale=True, layer_scale_init_value=1e-5, 
                 fork_feat=False,
                 init_cfg=None, 
                 pretrained=None, 
                 **kwargs):

        super().__init__()

        if not fork_feat:
            self.num_classes = num_classes
        self.fork_feat = fork_feat

        self.patch_embed = PatchEmbed(
            patch_size=in_patch_size, stride=in_stride, padding=in_pad, 
            in_chans=3, embed_dim=embed_dims[0])

        # set the main block in network
        network = []
        for i in range(len(layers)):
            stage = basic_blocks(embed_dims[i], i, layers, 
                                pool_size=pool_size, mlp_ratio=mlp_ratios[i],
                                act_layer=act_layer, norm_layer=norm_layer, 
                                drop_rate=drop_rate, 
                                drop_path_rate=drop_path_rate,
                                use_layer_scale=use_layer_scale, 
                                layer_scale_init_value=layer_scale_init_value)
            network.append(stage)
            if i >= len(layers) - 1:
                break
            if downsamples[i] or embed_dims[i] != embed_dims[i+1]:
                # downsampling between two stages
                network.append(
                    PatchEmbed(
                        patch_size=down_patch_size, stride=down_stride, 
                        padding=down_pad, 
                        in_chans=embed_dims[i], embed_dim=embed_dims[i+1]
                        )
                    )

        self.network = nn.ModuleList(network)


        # Classifier head
        self.norm = norm_layer(embed_dims[-1])
        self.head = nn.Linear(
            embed_dims[-1], num_classes) if num_classes > 0 \
            else nn.Identity()

        self.apply(self.cls_init_weights)


    # init for classification
    def cls_init_weights(self, m):
        if isinstance(m, nn.Linear):
            trunc_normal_(m.weight, std=.02)
            if isinstance(m, nn.Linear) and m.bias is not None:
                nn.init.constant_(m.bias, 0)

    def get_classifier(self):
        return self.head

    def reset_classifier(self, num_classes):
        self.num_classes = num_classes
        self.head = nn.Linear(
            self.embed_dim, num_classes) if num_classes > 0 else nn.Identity()

    def forward_embeddings(self, x):
        x = self.patch_embed(x)
        return x

    def forward_tokens(self, x):
        outs = []
        for idx, block in enumerate(self.network):
            x = block(x)
            if self.fork_feat and idx in self.out_indices:
                norm_layer = getattr(self, f'norm{idx}')
                x_out = norm_layer(x)
                outs.append(x_out)
        if self.fork_feat:
            # output the features of four stages for dense prediction
            return outs
        # output only the features of last layer for image classification
        return x

    def forward(self, x):
        # input embedding
        x = self.forward_embeddings(x)
        # through backbone
        x = self.forward_tokens(x)
        if self.fork_feat:
            # otuput features of four stages for dense prediction
            return x
        x = self.norm(x)
        cls_out = self.head(x.mean([-2, -1]))
        # for image classification
        return cls_out    


# ---------------
model_urls = {
    "poolformer_s12": "https://github.com/sail-sg/poolformer/releases/download/v1.0/poolformer_s12.pth.tar",
}


@register_model
def poolformer_s12(pretrained=False, **kwargs):
    """
    PoolFormer-S12 model, Params: 12M
    --layers: [x,x,x,x], numbers of layers for the four stages
    --embed_dims, --mlp_ratios: 
        embedding dims and mlp ratios for the four stages
    --downsamples: flags to apply downsampling or not in four blocks
    """
    layers = [2, 2, 6, 2]
    embed_dims = [64, 128, 320, 512]
    mlp_ratios = [4, 4, 4, 4]
    downsamples = [True, True, True, True]
    model = PoolFormer(
        layers, embed_dims=embed_dims, 
        mlp_ratios=mlp_ratios, downsamples=downsamples, 
        **kwargs)
    model.default_cfg = default_cfgs['poolformer_s']
    if pretrained:
        url = model_urls['poolformer_s12']
        checkpoint = torch.hub.load_state_dict_from_url(url=url, map_location="cpu", check_hash=True)
        model.load_state_dict(checkpoint)
    return model




if __name__ == "__main__": 
    x = torch.randn(1, 3, 224, 224) # input image 

    model = poolformer_s12()
    output = model(x)

    print(output.shape)