No gradient clipping in AdamW
Hi!

After moving from pretrained-bert to transformers I've noticed that the new AdamW optimizer does not perform gradient clipping, even though both BertAdam and OpenAIAdam used to do it. 

Also, in finetune_on_pregenerated example bias correction is turned off only for FusedAdam, but not for AdamW.
Yes, the LM fine-tuning example will be refactored.

Adding the removal of gradient clipping to the list of breaking changes, thanks.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
