Introduce lambda-based kernel API (#18541)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18541
Allow registering lambdas as c10 kernels.
Reviewed By: dzhulgakov
Differential Revision: D14653005
fbshipit-source-id: f867cc776b1339e83b7a2e1935f5cf924cfba44a
