Add option to keep tb_writer open after training is done.
Currently, `Trainer` automatically closes the tensorboard writer `tb_writer` after the training loop is done. Often I'll want to add extra observations, metrics, data, etc ... to the tensorboard event that requires the trained model (ie after the training loop is done). For example.

```
tb_writer = SummaryWriter(log_dir="some_dir")

# do some stuff with tb_writer before training

trainer = Trainer(
    model=model,
    args=training_args,
    train_dataset=train_dataset,
    eval_dataset=test_dataset,
    compute_metrics=compute_metrics,
    tb_writer=tb_writer,
)
trainer.train()

# do extra stuff with tb_writer after the model is trained (that requires a trained model)
```

## Your contribution

Submitted a PR: [Adds option to keep tb_writer open after training finishes](https://github.com/huggingface/transformers/pull/5328#issue-440856807)
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
