DropBlock

(a)原始输入图像

(b)绿色部分表示激活的特征单元，b图表示了随机dropout激活单元，但是这样dropout后，网络还会从drouout掉的激活单元附近学习到同样的信息

(c)绿色部分表示激活的特征单元，c图表示本文的DropBlock，通过dropout掉一部分相邻的整片的区域（比如头和脚），网络就会去注重学习狗的别的部位的特征，来实现正确分类，从而表现出更好的泛化。

code:

import torch
import torch.nn.functional as F
from torch import nn
import numpy as np

class DropBlock2D(nn.Module):
    r"""Randomly zeroes 2D spatial blocks of the input tensor.
    As described in the paper
    `DropBlock: A regularization method for convolutional networks`_ ,
    dropping whole blocks of feature map allows to remove semantic
    information as compared to regular dropout.
    Args:
        drop_prob (float): probability of an element to be dropped.
        block_size (int): size of the block to drop
    Shape:
        - Input: `(N, C, H, W)`
        - Output: `(N, C, H, W)`
    .. _DropBlock: A regularization method for convolutional networks:
       https://arxiv.org/abs/1810.12890
    """

    def __init__(self, drop_prob, block_size):
        super(DropBlock2D, self).__init__()

        self.drop_prob = drop_prob
        self.block_size = block_size

    def forward(self, x):
        # shape: (bsize, channels, height, width)

        assert x.dim() == 4, \
            "Expected input with 4 dimensions (bsize, channels, height, width)"

        if not self.training or self.drop_prob == 0.:
            return x
        else:
            # get gamma value
            gamma = self._compute_gamma(x)

            # sample mask
            mask = (torch.rand(x.shape[0], *x.shape[2:]) < gamma).float()

            # place mask on input device
            mask = mask.to(x.device)

            # compute block mask
            block_mask = self._compute_block_mask(mask)

            # apply block mask
            out = x * block_mask[:, None, :, :]

            # scale output
            out = out * block_mask.numel() / block_mask.sum()

            return out

    def _compute_block_mask(self, mask):
        block_mask = F.max_pool2d(input=mask[:, None, :, :],
                                  kernel_size=(self.block_size, self.block_size),
                                  stride=(1, 1),
                                  padding=self.block_size // 2)

        if self.block_size % 2 == 0:
            block_mask = block_mask[:, :, :-1, :-1]

        block_mask = 1 - block_mask.squeeze(1)

        return block_mask

    def _compute_gamma(self, x):
        return self.drop_prob / (self.block_size ** 2)

if __name__ == "__main__":

    # (bsize, n_feats, height, width)
    x = torch.rand(10, 2, 8, 8)
    drop_block = DropBlock2D(block_size=3, drop_prob=0.3)
    regularized_x = drop_block(x)
    print(regularized_x.shape)