[Documentation] Improve docs for hyper-parameter search
# 🚀 Improve documentation for hyper-parameter search

## Motivation

I was recently trying to implement a custom logging callback to use with `Trainer.hyperparameter_search(...)`, but I could not figure out how to access the current run's hyper-parameters to log them. After digging into the source code I discovered that they are available as `TrainerState.trial_params`, which is [currently undocumented](https://huggingface.co/transformers/master/main_classes/callback.html#trainerstate). To make it easier for future users I think the `trial_name` and `trial_params` properties of `TrainerState` should be documented.

## Your contribution

If the maintainers agree that these properties should be documented I'm happy to edit the docstring and submit a PR. Please let me know.

@sgugger 

Yes, please go ahead with a PR, thank you!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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