Improve pytorch examples for fp16
# What does this PR do?

When fp16 is True in pytorch training examples, use padding to multiple of 8, if the currently used data collator allows, to speed up training. If no collator was used, add one using the padding option.
Fixes #9752 


## Who can review?

 Trainer: @sgugger 
Thanks for addressing the comments, this is good to go IMO.
Pro-tip, if you edit your description to replace `Issue #9752` by `Fixes #9752`, the issue will be automatically closed when we merge this PR.