add torch.jit.is_scripting() api (#25263)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25263
This adds an api to return true in script and false in eager, which together with ignore allows guarding of not yet supported JIT features. Bikeshedding requested please.
cc zou3519
```
def foo():
if not torch.jit.is_scripting():
return torch.linear(...)
else:
return addmm(...)
```
Test Plan: Imported from OSS
Differential Revision: D17272443
Pulled By: eellison
fbshipit-source-id: de0f769c7eaae91de0007b98969183df93a91f42
