Fix F401 flake8 warning (x152 / 268).

This change is mostly autogenerated with:

    $ python -m autoflake --in-place --recursive examples templates transformers utils hubconf.py setup.py

I made minor changes in the generated diff.

transformers/modeling_encoder_decoder.py

@@ -18,7 +18,6 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 
 import logging
 import os
-import warnings
 
 import torch
 from torch import nn

transformers/modeling_gpt2.py

@@ -17,13 +17,9 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import collections
-import json
 import logging
 import math
 import os
-import sys
-from io import open
 
 import torch
 import torch.nn as nn

transformers/modeling_openai.py

@@ -17,12 +17,10 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import collections
 import json
 import logging
 import math
 import os
-import sys
 from io import open
 
 import torch

transformers/modeling_t5.py

@@ -18,12 +18,9 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 
 import copy
 import itertools
-import json
 import logging
 import math
 import os
-import sys
-from io import open
 
 import torch
 import torch.nn.functional as F

transformers/modeling_tf_bert.py

@@ -17,12 +17,8 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import json
 import logging
-import math
-import os
 import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_ctrl.py

@@ -18,9 +18,6 @@
 from __future__ import absolute_import, division, print_function, unicode_literals
 
 import logging
-import os
-import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_distilbert.py

@@ -16,13 +16,9 @@
 """
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import copy
 import itertools
-import json
 import logging
 import math
-import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_gpt2.py

@@ -17,13 +17,7 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import collections
-import json
 import logging
-import math
-import os
-import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_openai.py

@@ -17,13 +17,7 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import collections
-import json
 import logging
-import math
-import os
-import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_transfo_xl.py

@@ -18,13 +18,7 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import collections
-import json
 import logging
-import math
-import os
-import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_transfo_xl_utilities.py

@@ -16,7 +16,6 @@
 """ A TF 2.0 Adaptive Softmax for Transformer XL model.
 """
 
-from collections import defaultdict
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_xlm.py

@@ -19,7 +19,6 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 import itertools
 import logging
 import math
-import os
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_tf_xlnet.py

@@ -17,12 +17,8 @@
 """
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import json
 import logging
-import math
-import os
 import sys
-from io import open
 
 import numpy as np
 import tensorflow as tf

transformers/modeling_transfo_xl.py

@@ -20,13 +20,7 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import collections
-import json
 import logging
-import math
-import os
-import sys
-from io import open
 
 import torch
 import torch.nn as nn

transformers/modeling_transfo_xl_utilities.py

@@ -17,7 +17,6 @@
     Directly adapted from https://github.com/kimiyoung/transformer-xl.
 """
 
-from collections import defaultdict
 
 import numpy as np
 import torch

transformers/modeling_utils.py

@@ -17,11 +17,8 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import copy
-import json
 import logging
 import os
-from io import open
 
 import six
 import torch

transformers/modeling_xlnet.py

@@ -17,12 +17,9 @@
 """
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import json
 import logging
 import math
-import os
 import sys
-from io import open
 
 import torch
 from torch import nn

transformers/tests/configuration_common_test.py

@@ -16,7 +16,6 @@ from __future__ import absolute_import, division, print_function
 
 import json
 import os
-import tempfile
 import unittest
 
 from .tokenization_tests_commons import TemporaryDirectory

transformers/tests/modeling_auto_test.py

@@ -15,7 +15,6 @@
 from __future__ import absolute_import, division, print_function
 
 import logging
-import shutil
 import unittest
 
 from transformers import is_torch_available

transformers/tests/modeling_common_test.py

@@ -47,7 +47,6 @@ if is_torch_available():
     )
 
 if sys.version_info[0] == 2:
-    import cPickle as pickle
 
     class TemporaryDirectory(object):
         """Context manager for tempfile.mkdtemp() so it's usable with "with" statement."""
@@ -61,8 +60,6 @@ if sys.version_info[0] == 2:
 
 
 else:
-    import pickle
-
     TemporaryDirectory = tempfile.TemporaryDirectory
     unicode = str