Make Keras 2 available for TensorFlow 2.16.1

[ethanbb]

Description

This provides a way to continue using Keras 2 with TensorFlow 2.16.1 (see #1355) by following the instructions here: https://github.com/tensorflow/tensorflow/releases/tag/v2.16.1 While sticking with 2.15.0 should still work for most people, I had an annoying CUDA situation which was only fixed by upgrading, so I wanted to see if I could make 2.16 work.

It's somewhat hacky (especially in nn_models.py), but since the long-term goal is to move away from TensorFlow anyway, I thought that was OK. No worries if you don't want to merge this, though.

It requires a matching version of tf-keras to be installed alongside tensorflow, but this package is only available for 2.15 and 2.16, so it won't work on Windows where tensorflow 2.10 is the maximum. So there would have to be some OS-dependent logic in the environment specification; I didn't touch any of that and I think it might be only possible for the conda recipe (i.e. not in environment.yml).

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

Has your PR been tested?

Ran caimanmanager test and caimanmanager demotest.