use torch.cummax in corner_pool for torch 1.5+ (#390)

* synchronize from mmdetection

* fix parrots

mmcv/ops/corner_pool.py

+import torch
 from torch import nn
 from torch.autograd import Function
 
@@ -98,10 +99,27 @@ class CornerPool(nn.Module):
         'top': TopPoolFunction,
     }
 
+    cummax_dim_flip = {
+        'bottom': (2, False),
+        'left': (3, True),
+        'right': (3, False),
+        'top': (2, True),
+    }
+
     def __init__(self, mode):
         super(CornerPool, self).__init__()
         assert mode in self.pool_functions
+        self.mode = mode
         self.corner_pool = self.pool_functions[mode]
 
     def forward(self, x):
+        if torch.__version__ != 'parrots' and torch.__version__ >= '1.5.0':
+            dim, flip = self.cummax_dim_flip[self.mode]
+            if flip:
+                x = x.flip(dim)
+            pool_tensor, _ = torch.cummax(x, dim=dim)
+            if flip:
+                pool_tensor = pool_tensor.flip(dim)
+            return pool_tensor
+        else:
             return self.corner_pool.apply(x)