Ray Tune Integration Bug Fixes
# What does this PR do?
Fixes resource allocation and checkpointing bugs with the Ray Tune `hyperparameter_search` integration.

@sgugger @richardliaw @krfricke 
@amogkam There is an `s` missing in line 202 in `src/transformers/integrations.py` in `{kwargs['keep_checkpoint_num']}` which should be `{kwargs['keep_checkpoints_num']}`, which is causing the logger to crash instead of just a warning. Thanks for the fixes btw!
