Avoid closure around kernel (#16165)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16165
Store kernels as direct function pointers instead of std::function.
Using direct function pointers avoids a performance risk std::function would introduce.
Reviewed By: ezyang
Differential Revision: D13738627
fbshipit-source-id: a348906c8a201436699681980a82ca95065a06a0
