make FusionCallback refer to specified GraphFuser context (#41560)
Summary:
Fixes issue where
- top level fuser's block_ was captured by callback due to [&] capture,
- recursive/nested fusers would compare erroneously to top-level block_ instead of own block_
Closes (https://github.com/pytorch/pytorch/issues/39810)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41560
Reviewed By: Krovatkin
Differential Revision: D22583196
Pulled By: wconstab
fbshipit-source-id: 8f543cd9ea00e116cf3e776ab168cdd9fed69632
