Use 12xlarge for nightly cpp doc generation job (#86859)
The job starts to run out of memory a lot recently https://hud.pytorch.org/failure/Process%20completed%20with%20exit%20code%20137. Probably more and more docs are added, so this ups the runner for cpp doc nightly from 4xlarge to the next tier of 12xlarge. This also choose the smaller runner of 2xlarge for python and functorch docs (may be linux.large is good enough for them?)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86859
Approved by: https://github.com/malfet
