Make TensorFlow and PyTorch optional requirements

[devdrian]

Both, PyTorch and TensorFlow, are large packages and not required for all parts of UQ360. Even more so, if someone just intends to use the metrics provided by UQ360, the requirements could introduce some conflicts, e.g., if the user requires TensorFlow>=2.6.

Could we make both requirements optional, e.g., via extras_require{} in the setup.py? The downside would be, as far as I see it, that the default installation pip install uq360 would only install the minimal version and the full version would only be available via, e.g., pip install uq360[full].

In case you'd find this feature useful, I'd be happy to create a merge request!

[pronics2004]

Hi @devdrian This will be quite useful. You are welcome to create a merge request!

Please refer to CONTRIBUTING.md as you create the PR.