docs(optimization): document num_cycles, not the nonexistent num_periods

[simpleqt]

get_cosine_schedule_with_warmup's docstring documents a num_periods parameter. No such parameter exists — the signature and the cosine closure use num_cycles (and the hard-restart sibling documents num_cycles correctly). One-line docstring fix.

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[github-actions]

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[simpleqt]

Closing — pooled into #14715 combined docs PR to reduce review overhead. All fixes are preserved in the combined branch.

[simpleqt]

Correction: my closing comment pointed to #14715 — that number was wrong. The live pooled PR is #14757. Note this PR's num_cycles fix is not part of the pool anymore: it is already fixed upstream on main, so #14757 was rebased without it.