Cover more Python and PyTorch versions in unit tests matrix

[cofri]

The unit test environment only deals with Python 3.7 to 3.9 and with PyTorch 1.7.0.
We must cover a larger range of versions, especially the latest Python and PyTorch releases.

tox environments must be extended and the GitHub actions must run multiple combinations of Python/PyTorch versions to ensure the good behaviour of the library.