[JIT] Disallow plain Dict type annotation without arg (#44334)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44334
**Summary**
This commit detects and prohibits the case in which `typing.Dict` is
used as an annotation without type arguments (i.e. `typing.Dict[K, V]`).
At present, `typing.Dict` is always assumed to have two arguments, and
when it is used without them, `typing.Dict.__args__` is nonempty and
contains some `typing.TypeVar` instances, which have no JIT type equivalent.
Consequently, trying to convert `typing.Dict` to a JIT type results in
a `c10::DictType` with `nullptr` for its key and value types, which can cause
a segmentation fault.
This is fixed by returning a `DictType` from
`jit.annotations.try_ann_to_type` only if the key and value types are converted
successfully to a JIT type and returning `None` otherwise.
**Test Plan**
This commit adds a unit test to `TestDict` that tests the plain `Dict`
annotations throw an error.
**Fixes**
This commit closes #43530.
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D23610766
Pulled By: SplitInfinity
fbshipit-source-id: 036b10eff6e3206e0da3131cfb4997d8189c4fec
