新型卷积 | 涨点神器！利用Involution可构建新一代神经网络！(文末获取论文与源码)

from torch.autograd import Function
import torch
from torch.nn.modules.utils import _pair
import torch.nn.functional as F
import torch.nn as nn
from mmcv.cnn import ConvModule
from collections import namedtuple
import cupy
from string import Template
Stream = namedtuple('Stream', ['ptr'])

def Dtype(t):
    if isinstance(t, torch.cuda.FloatTensor):
        return 'float'
    elif isinstance(t, torch.cuda.DoubleTensor):
        return 'double'

@cupy._util.memoize(for_each_device=True)
def load_kernel(kernel_name, code, **kwargs):
    code = Template(code).substitute(**kwargs)
    kernel_code = cupy.cuda.compile_with_cache(code)
    return kernel_code.get_function(kernel_name)

CUDA_NUM_THREADS = 1024
kernel_loop = '''
#define CUDA_KERNEL_LOOP(i, n)                        \
  for (int i = blockIdx.x * blockDim.x + threadIdx.x; \
      i < (n);                                       \
      i += blockDim.x * gridDim.x)
'''

def GET_BLOCKS(N):
    return (N + CUDA_NUM_THREADS - 1) // CUDA_NUM_THREADS
_involution_kernel = kernel_loop + '''
extern "C"
__global__ void involution_forward_kernel(
const ${Dtype}* bottom_data, const ${Dtype}* weight_data, ${Dtype}* top_data) {
  CUDA_KERNEL_LOOP(index, ${nthreads}) {
    const int n = index / ${channels} / ${top_height} / ${top_width};
    const int c = (index / ${top_height} / ${top_width}) % ${channels};
    const int h = (index / ${top_width}) % ${top_height};
    const int w = index % ${top_width};
    const int g = c / (${channels} / ${groups});
    ${Dtype} value = 0;
    #pragma unroll
    for (int kh = 0; kh < ${kernel_h}; ++kh) {
      #pragma unroll
      for (int kw = 0; kw < ${kernel_w}; ++kw) {
        const int h_in = -${pad_h} + h * ${stride_h} + kh * ${dilation_h};
        const int w_in = -${pad_w} + w * ${stride_w} + kw * ${dilation_w};
        if ((h_in >= 0) && (h_in < ${bottom_height})
          && (w_in >= 0) && (w_in < ${bottom_width})) {
          const int offset = ((n * ${channels} + c) * ${bottom_height} + h_in)
            * ${bottom_width} + w_in;
          const int offset_weight = ((((n * ${groups} + g) * ${kernel_h} + kh) * ${kernel_w} + kw) * ${top_height} + h)
            * ${top_width} + w;
          value += weight_data[offset_weight] * bottom_data[offset];
        }
      }
    }
    top_data[index] = value;
  }
}
'''
_involution_kernel_backward_grad_input = kernel_loop + '''
extern "C"
__global__ void involution_backward_grad_input_kernel(
    const ${Dtype}* const top_diff, const ${Dtype}* const weight_data, ${Dtype}* const bottom_diff) {
  CUDA_KERNEL_LOOP(index, ${nthreads}) {
    const int n = index / ${channels} / ${bottom_height} / ${bottom_width};
    const int c = (index / ${bottom_height} / ${bottom_width}) % ${channels};
    const int h = (index / ${bottom_width}) % ${bottom_height};
    const int w = index % ${bottom_width};
    const int g = c / (${channels} / ${groups});
    ${Dtype} value = 0;
    for (int kh = 0; kh < ${kernel_h}; ++kh) {
      for (int kw = 0; kw < ${kernel_w}; ++kw) {
        const int h_out_s = h + ${pad_h} - kh * ${dilation_h};
        const int w_out_s = w + ${pad_w} - kw * ${dilation_w};
        if (((h_out_s % ${stride_h}) == 0) && ((w_out_s % ${stride_w}) == 0)) {
          const int h_out = h_out_s / ${stride_h};
          const int w_out = w_out_s / ${stride_w};
          if ((h_out >= 0) && (h_out < ${top_height})
                && (w_out >= 0) && (w_out < ${top_width})) {
            const int offset = ((n * ${channels} + c) * ${top_height} + h_out)
                  * ${top_width} + w_out;
            const int offset_weight = ((((n * ${groups} + g) * ${kernel_h} + kh) * ${kernel_w} + kw) * ${top_height} + h_out)
                  * ${top_width} + w_out;
            value += weight_data[offset_weight] * top_diff[offset];
          }
        }
      }
    }
    bottom_diff[index] = value;
  }
}
'''
_involution_kernel_backward_grad_weight = kernel_loop + '''
extern "C"
__global__ void involution_backward_grad_weight_kernel(
    const ${Dtype}* const top_diff, const ${Dtype}* const bottom_data, ${Dtype}* const buffer_data) {
  CUDA_KERNEL_LOOP(index, ${nthreads}) {
    const int h = (index / ${top_width}) % ${top_height};
    const int w = index % ${top_width};
    const int kh = (index / ${kernel_w} / ${top_height} / ${top_width})
          % ${kernel_h};
    const int kw = (index / ${top_height} / ${top_width}) % ${kernel_w};
    const int h_in = -${pad_h} + h * ${stride_h} + kh * ${dilation_h};
    const int w_in = -${pad_w} + w * ${stride_w} + kw * ${dilation_w};
    if ((h_in >= 0) && (h_in < ${bottom_height})
          && (w_in >= 0) && (w_in < ${bottom_width})) {
      const int g = (index / ${kernel_h} / ${kernel_w} / ${top_height} / ${top_width}) % ${groups};
      const int n = (index / ${groups} / ${kernel_h} / ${kernel_w} / ${top_height} / ${top_width}) % ${num};
      ${Dtype} value = 0;
      for (int c = g * (${channels} / ${groups}); c < (g + 1) * (${channels} / ${groups}); ++c) {
        const int top_offset = ((n * ${channels} + c) * ${top_height} + h)
              * ${top_width} + w;
        const int bottom_offset = ((n * ${channels} + c) * ${bottom_height} + h_in)
              * ${bottom_width} + w_in;
        value += top_diff[top_offset] * bottom_data[bottom_offset];
      }
      buffer_data[index] = value;
    } else {
      buffer_data[index] = 0;
    }
  }
}
'''

class _involution(Function):
    @staticmethod
    def forward(ctx, input, weight, stride, padding, dilation):
        assert input.dim() == 4 and input.is_cuda
        assert weight.dim() == 6 and weight.is_cuda
        batch_size, channels, height, width = input.size()
        kernel_h, kernel_w = weight.size()[2:4]
        output_h = int((height + 2 * padding[0] - (dilation[0] * (kernel_h - 1) + 1)) / stride[0] + 1)
        output_w = int((width + 2 * padding[1] - (dilation[1] * (kernel_w - 1) + 1)) / stride[1] + 1)

        output = input.new(batch_size, channels, output_h, output_w)
        n = output.numel()

        with torch.cuda.device_of(input):
            f = load_kernel('involution_forward_kernel', _involution_kernel, Dtype=Dtype(input), nthreads=n,
                            num=batch_size, channels=channels, groups=weight.size()[1],
                            bottom_height=height, bottom_width=width,
                            top_height=output_h, top_width=output_w,
                            kernel_h=kernel_h, kernel_w=kernel_w,
                            stride_h=stride[0], stride_w=stride[1],
                            dilation_h=dilation[0], dilation_w=dilation[1],
                            pad_h=padding[0], pad_w=padding[1])
            f(block=(CUDA_NUM_THREADS,1,1),
              grid=(GET_BLOCKS(n),1,1),
              args=[input.data_ptr(), weight.data_ptr(), output.data_ptr()],
              stream=Stream(ptr=torch.cuda.current_stream().cuda_stream))

        ctx.save_for_backward(input, weight)
        ctx.stride, ctx.padding, ctx.dilation = stride, padding, dilation
        return output
    
    @staticmethod
    def backward(ctx, grad_output):
        assert grad_output.is_cuda and grad_output.is_contiguous()
        input, weight = ctx.saved_tensors
        stride, padding, dilation = ctx.stride, ctx.padding, ctx.dilation

        batch_size, channels, height, width = input.size()
        kernel_h, kernel_w = weight.size()[2:4]
        output_h, output_w = grad_output.size()[2:]

        grad_input, grad_weight = None, None

        opt = dict(Dtype=Dtype(grad_output),
                   num=batch_size, channels=channels, groups=weight.size()[1],
                   bottom_height=height, bottom_width=width,
                   top_height=output_h, top_width=output_w,
                   kernel_h=kernel_h, kernel_w=kernel_w,
                   stride_h=stride[0], stride_w=stride[1],
                   dilation_h=dilation[0], dilation_w=dilation[1],
                   pad_h=padding[0], pad_w=padding[1])

        with torch.cuda.device_of(input):
            if ctx.needs_input_grad[0]:
                grad_input = input.new(input.size())

                n = grad_input.numel()
                opt['nthreads'] = n

                f = load_kernel('involution_backward_grad_input_kernel',
                                _involution_kernel_backward_grad_input, **opt)
                f(block=(CUDA_NUM_THREADS,1,1),
                  grid=(GET_BLOCKS(n),1,1),
                  args=[grad_output.data_ptr(), weight.data_ptr(), grad_input.data_ptr()],
                  stream=Stream(ptr=torch.cuda.current_stream().cuda_stream))

            if ctx.needs_input_grad[1]:
                grad_weight = weight.new(weight.size())

                n = grad_weight.numel()
                opt['nthreads'] = n

                f = load_kernel('involution_backward_grad_weight_kernel',
                                _involution_kernel_backward_grad_weight, **opt)
                f(block=(CUDA_NUM_THREADS,1,1),
                  grid=(GET_BLOCKS(n),1,1),
                  args=[grad_output.data_ptr(), input.data_ptr(), grad_weight.data_ptr()],
                  stream=Stream(ptr=torch.cuda.current_stream().cuda_stream))

        return grad_input, grad_weight, None, None, None
 

def _involution_cuda(input, weight, bias=None, stride=1, padding=0, dilation=1):
    """ involution kernel
    """
    assert input.size(0) == weight.size(0)
    assert input.size(-2)//stride == weight.size(-2)
    assert input.size(-1)//stride == weight.size(-1)
    if input.is_cuda:
        out = _involution.apply(input, weight, _pair(stride), _pair(padding), _pair(dilation))
        if bias is not None:
            out += bias.view(1,-1,1,1)
    else:
        raise NotImplementedError
    return out


class involution(nn.Module):

    def __init__(self,
                 channels,
                 kernel_size,
                 stride):
        super(involution, self).__init__()
        self.kernel_size = kernel_size
        self.stride = stride
        self.channels = channels
        reduction_ratio = 4
        self.group_channels = 16
        self.groups = self.channels // self.group_channels
        self.conv1 = ConvModule(
            in_channels=channels,
            out_channels=channels // reduction_ratio,
            kernel_size=1,
            conv_cfg=None,
            norm_cfg=dict(type='BN'),
            act_cfg=dict(type='ReLU'))
        self.conv2 = ConvModule(
            in_channels=channels // reduction_ratio,
            out_channels=kernel_size**2 * self.groups,
            kernel_size=1,
            stride=1,
            conv_cfg=None,
            norm_cfg=None,
            act_cfg=None)
        if stride > 1:
            self.avgpool = nn.AvgPool2d(stride, stride)

    def forward(self, x):
        weight = self.conv2(self.conv1(x if self.stride == 1 else self.avgpool(x)))
        b, c, h, w = weight.shape
        weight = weight.view(b, self.groups, self.kernel_size, self.kernel_size, h, w)
        out = _involution_cuda(x, weight, stride=self.stride, padding=(self.kernel_size-1)//2)
        return out