Pass existing tensorboard SummaryWriter to Trainer. 
# Feature request
The [`Trainer`](https://github.com/huggingface/transformers/blob/41750a6cff55e401364568868d619747de3db037/src/transformers/trainer.py#L102) class should let us pass our own `tb_writer` (Currently, a new `tb_writer` is created using the `args.logging_dir` argument if it exists).

## Motivation
This let's us have more detailed logs since we can add add things like hyper-params, histographs, and whatnot if we choose so.