python3Packages.torch: package NNPACK separately

[SomeoneSerge]

Issue description

Torch needs to be rid of fetchSubmodules = true (they prevent dependency injection; #239291, #277789, #276585), but in particular in #285249 we encountered the need to patch and configure NNPACK (unconditionally downloads dependencies from the internet without any error handling, fails configuration phase by running python with an invalid PYTHONPATH). Doing so inside the torch derivation would be cumbersome and doesn't scale (many more git-submodule time bombs). We should handle NNPACK once, without adding complexity to the torch expression. We should also ensure we can pass cmakeFlags to torch (currently obstructed by the setup.py wrapper)

References

#239211

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.