Better hyperparameter tensorboard logging in Trainer.
# 🚀 Feature request

`Trainer` should write `train_batch_size` and `eval_batch_size` to tensorboard. Currently only `per_device_` batch sizes are logged as hyperparamters, which means unless you know how many gpus you trained on (which is also not logged), you can't know the actual batch sizes used in training.

## Your contribution
 
Submitted a PR: [Adds train_batch_size, eval_batch_size, and n_gpu to to_sanitized_dict output for logging.](https://github.com/huggingface/transformers/pull/5331#issue-440857548)
