module 'tensorflow_core._api.v2.config' has no attribute 'list_physical_devices'
# 🐛 Bug

A bunch of tf tests fail with:

```module 'tensorflow_core._api.v2.config' has no attribute 'list_physical_devices'```

```
ERROR tests/test_benchmark_tf.py - AttributeError: module 'tensorflow_core._api.v2.config' has no attribute 'list_physical_devices'
ERROR tests/test_benchmark_tf.py - AttributeError: module 'tensorflow_core._api.v2.config' has no attribute 'list_physical_devices'
ERROR tests/test_benchmark_tf.py - AttributeError: module 'tensorflow_core._api.v2.config' has no attribute 'list_physical_devices'
ERROR tests/test_benchmark_tf.py - AttributeError: module 'tensorflow_core._api.v2.config' has no attribute 'list_physical_devices'
```

`tf.config.list_physical_devices` seems to be added in tf-2.1, so unless `transformers` starts to require tf >= 2.1, this breaks for tf < 2.1.

Depending on what you decide I can send a PR to fix this with either:

a. require  tf-2.1+ (simplest)
b. write a wrapper `list_physical_devices` that uses `tf.config.experimental.list_physical_devices` for tf < 2.1, and `tf.config.list_physical_devices` otherwise, and switch to using it.
c. do nothing?

## Environment info

```
- `transformers` version: 3.0.2
- Platform: Linux-4.15.0-109-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.5
- PyTorch version (GPU?): 1.5.1 (True)
- Tensorflow version (GPU?): 2.0.1 (False)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>
```

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.
