[JIT] add freeze to docs (#47120)
Summary:
freeze was temporarily renamed to _freeze in a reorg, and then removed from doc [here](https://github.com/pytorch/pytorch/pull/43473). add it back to docs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47120
Reviewed By: suo
Differential Revision: D24650712
Pulled By: eellison
fbshipit-source-id: 399e31586b8093de66937ba1266007ee291f509e
