Reimplement cc_attention using pure pytorch (open-mmlab#1201)

* Reimplement cc_attention using pure pytorch

* fix: avoid BC-Breaking

* delete cc_attention related cpp and cuda files

* delete cc_attention related lines in pybind.cpp

* make out Tensor contiguous.

* remove unneeded lines.

* Update mmcv/ops/cc_attention.py

Co-authored-by: Zaida Zhou <[email protected]>

* Update TestCrissCrossAttention

* passing pre-commit

* Update docstring of CrissCrossAttention

* Update docstring of CrissCrossAttention

* Update mmcv/ops/cc_attention.py

Co-authored-by: Zaida Zhou <[email protected]>

* [docs]polish the docstring

* [Docs] Polish the docstring

Co-authored-by: Zaida Zhou <[email protected]>

Co-authored-by: Zaida Zhou <[email protected]>

mmcv/ops/cc_attention.py

@@ -2,92 +2,78 @@
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
-from torch.autograd.function import once_differentiable
 
 from mmcv.cnn import PLUGIN_LAYERS, Scale
-from ..utils import ext_loader
 
-ext_module = ext_loader.load_ext(
-    '_ext', ['ca_forward', 'ca_backward', 'ca_map_forward', 'ca_map_backward'])
 
+def NEG_INF_DIAG(n, device):
+    """Returns a diagonal matrix of size [n, n].
 
-class CAWeightFunction(torch.autograd.Function):
+    The diagonal are all "-inf". This is for avoiding calculating the
+    overlapped element in the Criss-Cross twice.
+    """
+    return torch.diag(torch.tensor(float('-inf')).to(device).repeat(n), 0)
 
-    @staticmethod
-    def symbolic(g, t, f):
-        return g.op('mmcv::MMCVCAWeight', t, f)
 
-    @staticmethod
-    def forward(ctx, t, f):
-        n, c, h, w = t.size()
-        weight = torch.zeros(n, h + w - 1, h, w).to(t.device)
-        ext_module.ca_forward(t, f, weight)
-
-        ctx.save_for_backward(t, f)
-
-        return weight
-
-    @staticmethod
-    @once_differentiable
-    def backward(ctx, dw):
-        t, f = ctx.saved_tensors
-        dt = torch.zeros_like(t)
-        df = torch.zeros_like(f)
-        ext_module.ca_backward(dw, t, f, dt, df)
-        return dt, df
-
-
-class CAMapFunction(torch.autograd.Function):
-
-    @staticmethod
-    def symbolic(g, weight, v):
-        return g.op('mmcv::MMCVCAMap', weight, v)
-
-    @staticmethod
-    def forward(ctx, weight, v):
-        out = torch.zeros_like(v)
-        ext_module.ca_map_forward(weight, v, out)
-
-        ctx.save_for_backward(weight, v)
-
-        return out
-
-    @staticmethod
-    @once_differentiable
-    def backward(ctx, dout):
-        weight, v = ctx.saved_tensors
-        dw = torch.zeros_like(weight)
-        dv = torch.zeros_like(v)
-        ext_module.ca_map_backward(dout, weight, v, dw, dv)
+@PLUGIN_LAYERS.register_module()
+class CrissCrossAttention(nn.Module):
+    """Criss-Cross Attention Module.
 
-        return dw, dv
+    .. note::
+        Before v1.3.13, we use a CUDA op. Since v1.3.13, we switch
+        to a pure PyTorch and equivalent implementation. For more
+        details, please refer to https://github.com/open-mmlab/mmcv/pull/1201.
 
+        Speed comparison for one forward pass
 
-ca_weight = CAWeightFunction.apply
-ca_map = CAMapFunction.apply
+        - Input size: [2,512,97,97]
+        - Device: 1 NVIDIA GeForce RTX 2080 Ti
 
+        +-----------------------+---------------+------------+---------------+
+        |                       |PyTorch version|CUDA version|Relative speed |
+        +=======================+===============+============+===============+
+        |with torch.no_grad()   |0.00554402 s   |0.0299619 s |5.4x           |
+        +-----------------------+---------------+------------+---------------+
+        |no with torch.no_grad()|0.00562803 s   |0.0301349 s |5.4x           |
+        +-----------------------+---------------+------------+---------------+
 
-@PLUGIN_LAYERS.register_module()
-class CrissCrossAttention(nn.Module):
-    """Criss-Cross Attention Module."""
+    Args:
+        in_channels (int): Channels of the input feature map.
+    """
 
     def __init__(self, in_channels):
-        super(CrissCrossAttention, self).__init__()
+        super().__init__()
         self.query_conv = nn.Conv2d(in_channels, in_channels // 8, 1)
         self.key_conv = nn.Conv2d(in_channels, in_channels // 8, 1)
         self.value_conv = nn.Conv2d(in_channels, in_channels, 1)
         self.gamma = Scale(0.)
         self.in_channels = in_channels
 
     def forward(self, x):
-        proj_query = self.query_conv(x)
-        proj_key = self.key_conv(x)
-        proj_value = self.value_conv(x)
+        """forward function of Criss-Cross Attention.
+
+        Args:
+            x (Tensor): Input feature. \
+                shape (batch_size, in_channels, height, width)
+        Returns:
+            Tensor: Output of the layer, with shape of \
+            (batch_size, in_channels, height, width)
+        """
+        B, C, H, W = x.size()
+        query = self.query_conv(x)
+        key = self.key_conv(x)
+        value = self.value_conv(x)
+        energy_H = torch.einsum('bchw,bciw->bwhi', query, key) + NEG_INF_DIAG(
+            H, query.device)
+        energy_H = energy_H.transpose(1, 2)
+        energy_W = torch.einsum('bchw,bchj->bhwj', query, key)
+        attn = F.softmax(
+            torch.cat([energy_H, energy_W], dim=-1), dim=-1)  # [B,H,W,(H+W)]
+        out = torch.einsum('bciw,bhwi->bchw', value, attn[..., :H])
+        out += torch.einsum('bchj,bhwj->bchw', value, attn[..., H:])
 
-        energy = ca_weight(proj_query, proj_key)
-        attention = F.softmax(energy, 1)
-        out = ca_map(attention, proj_value)
         out = self.gamma(out) + x
+        out = out.contiguous()
 
         return out