Remove deprecation warning TF2.2
`strategy.experimental_run_v2()` is deprecated in favor of `strategy.run()`
https://github.com/tensorflow/tensorflow/blob/2b96f3662bd776e277f86997659e61046b56c315/tensorflow/python/distribute/distribute_lib.py#L953-L957

Fix #4992 
The only CI message is :

```
would reformat /home/circleci/transformers/src/transformers/trainer_tf.py
Oh no! 💥 💔 💥
1 file would be reformatted, 299 files would be left unchanged.
```

How can I see what to change to fit the code style ?<|||||>Thanks for the PR, but merging this, will remove the trainer from being compliant with TF <= 2.1. Which is not what we want yet. But we keep it here when we will be ready to do so ;)