Add Custom graph fusion (#18588)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18588
ghimport-source-id: f40df177af8b87c73f04bf337f478a62133284cf
Differential Revision: D14901297
Pulled By: bwasti
fbshipit-source-id: 1b6371a5175b3d63dad542b7cc22cb82e8c6cfd0
