Replace composite dispatch with `CompositeExplicitAutograd` (#64641)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64641
`sum`, `mean`, and `norm` were ported to structured kernels in #61642, #61643, and #62711,
respectively. Those PRs changed related overlads into composite kernels. However, their
dispatch section remained the same, when they really should be marked as
`CompositeExplicitAutograd`. This PR fixes this issue.
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D30867122
Pulled By: ezyang
fbshipit-source-id: b951aee41a3cab9ca546df826a285d60013e3b3a
