Revert D24714803: make duplicate def() calls an error in the dispatcher. Updating all fb operators to use the new dispatcher registration API
Test Plan: revert-hammer
Differential Revision:
D24714803 (https://github.com/pytorch/pytorch/commit/824f71069415babb2a699dbc0e9e95008decc8be)
Original commit changeset: c809aad8a698
fbshipit-source-id: fb2ada65f9fc00d965708d202bd9d050f13ef467
