[WIP] Pad to multiple of 8 for run mlm example
# What does this PR do?
This PR uses padding to a multiple of 8 in the run_mlm.py language modeling example, when fp16 is used. Since the DataCollatorForLanguageModeling did not initially accept the pad_to_multiple_of option, that functionality was added.
Fixes #10627


## Before submitting
- [x] Did you write any new necessary tests?


## Who can review?
@sgugger 
@sgugger I fixed an edge case in the `_collate_batch` method and added tests for that, but if it's ok I'd like to close this PR and open a fresh one.<|||||>Closing in favor of #11128