TF: future proof our keras imports
# What does this PR do?

On the release notes for [TF 2.11](https://github.com/tensorflow/tensorflow/releases/tag/v2.11.0), we can read:

```
tensorflow/python/keras code is a legacy copy of Keras since the TensorFlow v2.7 release, and 
will be deleted in the v2.12 release. Please remove any import of tensorflow.python.keras and 
use the public API with `from tensorflow import keras` or `import tensorflow as tf; tf.keras`.
```

On top of that, `hdf5_format` was moved from `keras.saving` to `keras.saving.legacy` in v2.11 (this one is not in the patch notes). 

This PR prepares us for v2.11 and beyond, while keeping retrocompatibility through gated imports.

Note: I've tested the v2.11 imports locally :) (the CI is running against v2.10)
Other than changing `save_attributes_to_hdf5_group` to `hdf5_format.save_attributes_to_hdf5_group`, no changes are needed now -- but they will be in v2.12, so might as well 🤷 <|||||>_The documentation is not available anymore as the PR was closed or merged._