Adds option to keep tb_writer open after training finishes
Set the `close_tb_writer` parameter to `False` in `Trainer.train()` to keep the tensorboard writer open.

```
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(close_tb_writer=False)

# you can now use tb_writer even after training!
```

Closes #5329 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=h1) Report
> Merging [#5328](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/393b8dc09a97197df1937a7e86c0c6b4ce69c7e9&el=desc) will **increase** coverage by `0.37%`.
> The diff coverage is `50.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5328/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5328      +/-   ##
==========================================
+ Coverage   77.54%   77.91%   +0.37%     
==========================================
  Files         138      138              
  Lines       24284    24284              
==========================================
+ Hits        18831    18922      +91     
+ Misses       5453     5362      -91     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.38% <50.00%> (ø)` | |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.26% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.37% <0.00%> (+0.72%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.62% <0.00%> (+1.47%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.77% <0.00%> (+2.22%)` | :arrow_up: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `95.23% <0.00%> (+38.09%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.38% <0.00%> (+68.46%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=footer). Last update [393b8dc...d390b0d](https://codecov.io/gh/huggingface/transformers/pull/5328?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@JetRunner I'm not exactly sure why the circleci build failed. I had the same result from this pull request https://github.com/huggingface/transformers/pull/5331, but I can't figure out how if at all it's related to my added code.<|||||>It’s not, don’t worry.
Let’s see if we can get that fixed. Otherwise, we’ll still be able to merge this PR since the CI failure has nothing to do with your added code.<|||||>@sgugger do you want to take a look at this?<|||||>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.
