[JIT] Add autocasting to freezing pass & enable autocast pass by default (#74178)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74178
Autocasting + freezing should reduce model size in some scenarios, since half-precision constants should be smaller than full-precision constants. This also enables the jit autocast pass by default, so `torch._C._jit_set_autocast_mode(True)` doesn't need to be set in order to enable autocasting.
Test Plan: Imported from OSS
Reviewed By: zou3519, eellison
Differential Revision: D34914245
Pulled By: davidberard98
fbshipit-source-id: 301f3669431feabbd695ebbdfc9c17bd1be3b565
(cherry picked from commit 0530cd365ae1f148910100a5c2981e80d04e4883)
