Revert "[refactor] Modified collections to collections.abc" (#987)

This reverts commit 0bcf4dae

.
Co-authored-by: Min Xu <min.xu.public@gmail.com>

fairscale/optim/grad_scaler.py

@@ -3,9 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-import collections.abc as abc
-from collections import defaultdict
-
+from collections import abc, defaultdict
 from enum import Enum
 import logging
 from typing import Any, Dict, List, Optional, Union

fairscale/utils/params.py

@@ -3,7 +3,7 @@
 # This source code is licensed under the BSD license found in the
 # LICENSE file in the root directory of this source tree.
 
-import collections.abc as abc
+from collections import abc
 from math import inf
 from typing import Any, Callable, Dict, List, Optional