Fix deprecation warning collections.abc (#312)

* Fix deprecation warning collections.abc

* Fix isort

mmcv/parallel/collate.py

 # Copyright (c) Open-MMLab. All rights reserved.
-import collections
+from collections.abc import Mapping, Sequence
 
 import torch
 import torch.nn.functional as F
@@ -20,7 +20,7 @@ def collate(batch, samples_per_gpu=1):
     3. cpu_only = False, stack = False, e.g., gt bboxes
     """
 
-    if not isinstance(batch, collections.Sequence):
+    if not isinstance(batch, Sequence):
         raise TypeError(f'{batch.dtype} is not supported.')
 
     if isinstance(batch[0], DataContainer):
@@ -73,10 +73,10 @@ def collate(batch, samples_per_gpu=1):
                 stacked.append(
                     [sample.data for sample in batch[i:i + samples_per_gpu]])
         return DataContainer(stacked, batch[0].stack, batch[0].padding_value)
-    elif isinstance(batch[0], collections.Sequence):
+    elif isinstance(batch[0], Sequence):
         transposed = zip(*batch)
         return [collate(samples, samples_per_gpu) for samples in transposed]
-    elif isinstance(batch[0], collections.Mapping):
+    elif isinstance(batch[0], Mapping):
         return {
             key: collate([d[key] for d in batch], samples_per_gpu)
             for key in batch[0]