[NNC] Remove all deferred expansion from Reductions (#47709)
Summary:
Refactors the ReduceOp node to remove the last remaining deferred functionality: completing the interaction between the accumulator buffer and the body. This fixes two issues with reductions:
1. Nodes inside the interaction could not be visited or modified, meaning we could generate bad code when the interaction was complex.
2. The accumulator load was created at expansion time and so could not be modified in some ways (ie. vectorization couldn't act on these loads).
This simplifies reduction logic quite a bit, but theres a bit more involved in the rfactor transform.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47709
Reviewed By: ZolotukhinM
Differential Revision: D24904220
Pulled By: nickgg
fbshipit-source-id: 159e5fd967d2d1f8697cfa96ce1bb5fc44920a40
