[JIT] make is_scripting a condvalue (#32871)
Summary:
Add `torch.jit.is_scripting` to the list of CondValues, or values that if they are an input to a if statement we only compile one side of the if. I'm not sure if we actually want this PR.
Pros:
- Makes it easier to add features that are not yet supported in TorchScript (like has_torch_function)
- The current idiom of writing `torch.jit.is_scripting` and factoring out the block to a function annotated with `torch.jit.ignore` is functionally equivalent and much more cumbersome
Cons:
- Makes it easier to add features that are not yet supported in TorchScript
- Perhaps is confusing as a reader what is being compiled. Potentially could give all caps name or otherwise change name to make it more visually stand out.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32871
Differential Revision: D19670383
Pulled By: eellison
fbshipit-source-id: 5257b0bd23c66f199d59a7f2c911e948301e5588
