Usability improvements for repo setup

[marcromeyn]

This PR does a few things:

• Gets rid of all the pytest.importorskip imports of tf/pytorch in our tests by moving this to the module-level. Therefore in the tests itself we can just import the frameworks in the default way.
• Splits the CI/CD into tensorflow-tests, torch-tests & pre-commit. This makes the results easier to parse and by re-using pre-commit we can make the results closer to what you would get locally.
• Introduces a Makefile with some common commands.

[gabrielspmoreira]

Sounds good to me. I just have a question on what will happen if the user has not both torch and tf installed, if you are removing the pytest.importorskip()

[marcromeyn]

Nothing changes, in the new setup we do this here: /~https://github.com/NVIDIA-Merlin/models/blob/usability/tests/tf/__init__.py#L18. So when tf/torch is not available it would never get to the individual test files.