preserve ignored function return value type (#25262)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25262
Preserve the type of ignore'd functions on serialization. Currently we first compile an ignore'd function with it's annotated type when first compiling, but do not preserve it. This is important for being able to compile models with not-yet-supported features in JIT.
```
torch.jit.ignore
def unsupported(x):
return x
def foo():
if not torch.jit._is_scripting():
return torch.linear(...)
else:
return unsupported(...)
```
Test Plan: Imported from OSS
Reviewed By: driazati
Differential Revision: D17199043
Pulled By: eellison
fbshipit-source-id: 1196fd94c207b9fbee1087e4b2ef7d4656a6647f
