Fix F401 flake8 warning (x88 / 116).

This change is mostly autogenerated with:

    $ python -m autoflake --in-place --recursive --remove-all-unused-imports --ignore-init-module-imports examples templates transformers utils hubconf.py setup.py

I made minor changes in the generated diff.

examples/run_bertology.py

@@ -26,8 +26,7 @@ from datetime import datetime
 
 import numpy as np
 import torch
-from torch.nn import CrossEntropyLoss, MSELoss
-from torch.utils.data import DataLoader, SequentialSampler, Subset, TensorDataset
+from torch.utils.data import DataLoader, SequentialSampler, Subset
 from torch.utils.data.distributed import DistributedSampler
 from tqdm import tqdm