update min tf requirements
All of the test suite is failing w/o this update - need to re-run `pip install -e .[dev]`

note that the failing tests don't show `"You need to run the TensorFlow trainer with at least the version 2.2.0, your version is {` anywhere so perhaps need some extra tweaks for the test fixtures, but then since non-tf tests fail too, the problem is in the core.

Not sure if perhaps this code needs to be replaced with an assert - then the error message will always be there regardless where it's used.
```
if parse(tf.__version__).release < (2, 2, 0):
    logger.info(
        "You need to run the TensorFlow trainer with at least the version 2.2.0, your version is {}".format(
            tf.__version__
        )
    )
    sys.exit(1)
```

Currently, if I run **any** test, including pytorch-only tests, with tf < 2.2 I get:

```
____________________________________________________________ ERROR collecting tests/test_benchmark.py ____________________________________________________________
tests/test_benchmark.py:6: in <module>
    from transformers import AutoConfig, is_torch_available
src/transformers/__init__.py:659: in <module>
    from .trainer_tf import TFTrainer
src/transformers/trainer_tf.py:34: in <module>
    sys.exit(1)
E   SystemExit: 1
======================================================================== warnings summary ========================================================================
```

To reproduce:

```
pip install tensorflow==2.0.1
pytest -ra tests/test_benchmark.py 
```

@jplu 


Hello !

Which tests do not pass precisely? They are all green for me.

For the log that is not displayed this is because no logger is set in `benchmark_test.py`. I will review that for later, thanks!<|||||>> Which tests do not pass precisely? They are all green for me.

Most (all?) tests. Try:
```
pip install tensorflow==2.0.1
pytest -ra tests/test_benchmark.py 
```

But unrelated to tests if the runtime requires `foo>=x.y.z`, then the requirements/setup need to require that exact version. A user may (1) already have `foo` installed and then runtime fails (2) a different package may require a lower version of `foo` and thus `pip` won't upgrade it to the latest version available.

> For the log that is not displayed this is because no logger is set in `benchmark_test.py`. I will review that for later, thanks!

That was just an example, as I said most, if not all tests fail with the same cryptic error.<|||||>OK, I see better now. Thanks!!

I have fixed this in another PR by updating the piece of code you raised by an assert inside the `__init__` of the TFTrainer and now your example works fine. We do not want to fix the TensorFlow version for the entire lib but only for the trainer, at least for now.<|||||>I understand.

Please let me know which PR if you'd like me to re-test, or when it gets merged and I will re-test then.

Thank you, @jplu <|||||>Now the fix is merged :)<|||||>Thank you for remembering to ping me. I re-tested with master and the tests now work with tensorflow==2.0.1 - thank you very much, @jplu 