Add type hints in mmcv/ops/active_rotated_filter.py (#2017)

* add typehints in ops-active-rotated-filter

* resolve typehints in ops-active-rotated-filter

mmcv/ops/active_rotated_filter.py

 # Copyright (c) OpenMMLab. All rights reserved.
+from typing import Tuple
+
 import torch
 from torch.autograd import Function
 from torch.autograd.function import once_differentiable
@@ -19,7 +21,8 @@ class ActiveRotatedFilterFunction(Function):
     """
 
     @staticmethod
-    def forward(ctx, input, indices):
+    def forward(ctx, input: torch.Tensor,
+                indices: torch.Tensor) -> torch.Tensor:
         """
         Args:
             input (torch.Tensor): Input features with shape
@@ -41,7 +44,7 @@ class ActiveRotatedFilterFunction(Function):
 
     @staticmethod
     @once_differentiable
-    def backward(ctx, grad_out):
+    def backward(ctx, grad_out: torch.Tensor) -> Tuple[torch.Tensor, None]:
         """
         Args:
             grad_output (torch.Tensor): The gradiant of output features