[SR] Avoid allocating rstd/mean in layer_norm (#73606)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73606
The single-output overload of `layer_norm` internally allocates two tensors. As an optimization, we previously added `static_runtime::layer_norm`. This variant of layer norm had two extra outputs to make the memory planner aware of these extra tensors. But these outputs were unused; it's actually better for us to avoid the allocation and associated computations entirely.
ghstack-source-id: 151394116
Test Plan: Existing unit tests
Reviewed By: hlu1
Differential Revision: D34562131
fbshipit-source-id: c6a6560e60db43b0b100aedc54ea4265acb347de
(cherry picked from commit 3bed52b6f688b93b9b032c3d2b4be68d08d8eb76)
