Update base for Update on "Add docstrings for save_on_cpu hooks"

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

Differential Revision: [D29990129](https://our.internmc.facebook.com/intern/diff/D29990129)

[ghstack-poisoned]