Use self._trial  to generate trial_name for Trainer.
# What does this PR do?

Generate trial name unless the trial is not None, and use `(self._trial or trial)` to generate trial name. 
Because [currently the `optuna` backend give a None trial when using DDP and rank != 0](https://github.com/huggingface/transformers/blob/v4.23.1/src/transformers/integrations.py#L193)

Related code:
https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/integrations.py#L160-L208


Or maybe the documentation should be changed.

https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/trainer.py#L2318-L2319

Who can review: 

* Trainer: @sgugger  
* optuna HPO: @sywangyi

_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger Please take a review