Run mlm pad to multiple for fp16
# 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 