[JIT] Cast return values of functions returning Any (#42259)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42259
**Summary**
This commit modifies IR generation to insert explicit cast that cast
each return value to `Any` when a function is annotated as returning `Any`.
This precludes the failure in type unification (see below) that caused
this issue.
Issue #41962 reported that the use of an `Any` return type in
combination with different code paths returning values of different
types causes a segmentation fault. This is because the exit transform
pass tries to unify the different return types, fails, but silently sets
the type of the if node to c10::nullopt. This causes problems later in
shape analysis when that type object is dereferenced.
**Test Plan**
This commit adds a unit test that checks that a function similar to the
one in #41962 can be scripted and executed.
**Fixes**
This commit fixes #41962.
Differential Revision: D22883244
Test Plan: Imported from OSS
Reviewed By: eellison, yf225
Pulled By: SplitInfinity
fbshipit-source-id: 523d002d846239df0222cd07f0d519956e521c5f
