NNPack from the got submodule will be compiled in if you request that during build (but I think you have to ask). It does have relatively low preference by default (less than mkldnn, but more than the THNN fallback) if you invoke the usual convolution functions.
The direct functions are not part of the official API (ie use at your own risk, might break between versions etc.), but grepping for nnpack in the tests will let you find how to invoke them.
Note that you’re off the beaten path with that - NNPack was removed from PyTorch at some point and I only added it back recently for the benefit of the Android port - where I only use inference).
Best regards
Thomas