chore: fixes in `Trainer` class docs (`compute_loss` & `hyperparameter_search`) (#44268)
* Fix docstring formatting in trainer.py
Fix formatting issues in docstring for hyperparameter search function.
* Fix typo in hyperparameters
* Fix truncated doc for trainer's `compute_loss`
Clarify the description of num_items_in_batch and fix minor typos.
* make style - whitespace