Fix tests of mixed precision now that experimental is deprecated
Thanks, @Rocketknight1 

I saw we have a few

```
tf.keras.mixed_precision.experimental.Policy
```

in `src/transformers/training_args_tf.py`. Maybe we also need to update these places? (I am OK if you prefer to do it in another PR, if the change is indeed necessary)<|||||>@ydshieh I'll fix them here too!<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>![image](https://user-images.githubusercontent.com/12240844/168825240-bbf3d9bf-f113-4621-9ccd-cdeb183c9695.png)

Doesn't this mean TF<=2.8 will fail in these lines?<|||||>(@Rocketknight1 )<|||||>For push/scheduled CI, I think it is fine, as the docker image is built with

```
RUN python3 -m pip install --no-cache-dir -U torch tensorflow
```
(right ..?)

I am going to print TF version in CI jobs.

Update: The latest docker image is built with 2.9

```
2022-05-17T01:34:05.0014817Z #10 15.23 Collecting tensorflow>=2.3
2022-05-17T01:34:05.0016257Z #10 15.24   Downloading tensorflow-2.9.0-cp38-cp38-manylinux_2_17_x86_64.manylinux2014_x86_64.whl (511.7 MB)
```<|||||>Yes, CI should be fine :D but users with older versions (which we support) will experience errors, right? Or is it backwards compatible?<|||||>You are right! At least for `src/transformers/training_args_tf.py`.

I am not very sure what's our policy regarding the test backward compatible (regarding the change in `tests/utils/test_modeling_tf_core.py`)