[Fix]Make sure the args tb_writer passed to the TensorBoardCallback works
Author: iamlockelightning
Pull request: yes

# What does this PR do?

Add an `if` check in the `on_train_begin` function of the `TensorBoardCallback`, to make sure the args `tb_writer` passed to the `__init__` won't be overwritten.


I wanted to pass a tb_writer to the `TensorBoardCallback`, so that I could continue using it after training. But the passed tb_writer didn't work.

```python
tb_writer = SummaryWriter(log_dir="some_dir")
cb = TensorBoardCallback(tb_writer)
trainer = Trainer(
    # not important
    callbacks=[cb],
)
trainer.train()
tb_writer.add_scalar("other metrics", 0.99, 0) # didn't work
```

After checking the code of `TensorBoardCallback`, I found that the tb_writer passed to the `__init__` would be overwritten by the `on_train_begin` function with a new one no matter what.

By adding an `if` statement before the init, this bug should be fixed.

```python
if self.tb_writer is None: # check
    self._init_summary_writer(args, log_dir)
```


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## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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https://github.com/huggingface/transformers/issues/5329 (a similar issue but is "wontfix" and out-of-date)

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## Who can review?

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