migrate tests from nbs to pytest

[deven367]

This PR is a WIP to remove nbdev from neuralforecast.

cc @goodwanghan @marcopeix

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[deven367]

tests for AutoModels have been moved with their respective models
cc @marcopeix

[marcopeix]

For the memory issues on MacOS, despite setting the MPS fallback variable, maybe we can try running the tests on a single thread? Don't know if they are running in parallel or not, but we could try setting n_workers=1 see if it solves the issue.

[deven367]

@marcopeix I just checked, pytest is single threaded by default