Add docstrings for save_on_cpu hooks (#62410)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62410
This PR adds docstrings for CPU hooks introduced in #61928.
Also uncomments the warning about pinned memory in CUDA semantics docs.
Depends on: #62361.
For now docstrings are an orphan page at https://docs-preview.pytorch.org/62410/generated/torch.autograd.graph.set_save_on_cpu_hooks.html#torch-autograd-graph-set-save-on-cpu-hooks
Test Plan: Imported from OSS
Reviewed By: soulitzer
Differential Revision: D29990129
Pulled By: Varal7
fbshipit-source-id: 7a98eeee6a0abb11e2c2d9169cd1aa35ad7ba3f4
