[JIT] Support scripting torch.is_autocast_enabled() (#81305)
This adds an `aten::is_autocast_enabled` op into the jit runtime so that
autocasting ops can be scripted and called from within jit.
Differential Revision: [D37901585](https://our.internmc.facebook.com/intern/diff/D37901585)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81305
Approved by: https://github.com/qihqi, https://github.com/eellison
