deprecation warning fixes (facebookresearch#2881)

Summary:
## What does this PR do?

Fixes:

- 2x `DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working`

- 1x `/fairseq/optim/adam.py:98: DeprecationWarning: invalid escape sequence \:`

This is with py38.

Pull Request resolved: facebookresearch#2881

Reviewed By: alexeib

Differential Revision: D24959633

Pulled By: myleott

fbshipit-source-id: ac563e194d5f07e3817de55729b0448366a6dc23

fairseq/optim/adam.py

@@ -5,7 +5,7 @@
 
 import logging
 import math
-from collections import Collection
+from collections.abc import Collection
 from dataclasses import dataclass, field
 from typing import List
 
@@ -95,7 +95,7 @@ def average_params(self):
 
 
 class Adam(torch.optim.Optimizer):
-    """Implements Adam algorithm.
+    r"""Implements Adam algorithm.
 
     This implementation is modified from torch.optim.Adam based on:
     `Fixed Weight Decay Regularization in Adam`

fairseq/optim/nag.py

@@ -3,7 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-from collections import Collection
+from collections.abc import Collection
 from dataclasses import dataclass, field
 from typing import List