dict type unification fix (#32185)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32185
Previously we would unify the contained types of dictionaries, however this breaks type safety.
```
torch.jit.script
def test(input: Dict[str, None], cond):
if cond:
out = input
else:
out: {"1": 1}
out["hi"] = 3
```
This would only occur if a dictionary is being re-assigned across an if condition with different contained types, which is pretty unlikely. I tested `model_backward_compatibility` for all fb models and this didn't break anything. This PR is a precursor to alias analysis changes.
Also fixes `Future` type unification. Because `Future` is an immutable type, it is okay to unify the contained type.
Test Plan: Imported from OSS
Differential Revision: D19398585
Pulled By: eellison
fbshipit-source-id: ebc8812cdf5b6dba37b1cfbc2edc7d8c467b258c
