Revert D21161361: [pytorch][PR] Revert "Revert D21089648: Put TORCH_LIBRARY in torch/library.h; add custom class API"
Test Plan: revert-hammer
Differential Revision:
D21161361
Original commit changeset: dca4192d3f7b
fbshipit-source-id: 311b6ab6169feb5e12ae8c959789f8f0acd5205d
