Allow pytorch backend when available in PyMC

[ricardoV94]

The barebones PyTorch backend should be in in the next PyTensor release and subsequent PyMC.

I think the best way to test it out and gather info on what Ops are most needed is to give access from nutpie, as it will:

1. Feel like experimental on experimental
2. Be tested by more code-savy people

What do you think?

[aseyboldt]

Coming up soon :-)

[aseyboldt]

Current work is here: #144