[JIT] Infer NamedTuple type attributes of nn.Modules correctly (#39116)
Summary:
**Summary**
This commit modifies type inference for `nn.Module` instance attributes
such that the type of a `NamedTuple` attribute is inferred correctly and
such that the field names of this `NamedTuple` instance can be used in
scripted methods. At present, the type of this attribute is inferred to be
`Tuple[T, U, ..., V]`, so the field must be referred to by index and
cannot be referred to by name.
**Test Plan**
This commit adds a unit test to test that a field of a `NamedTuple`
attribute can be referred to by name in a scripted method.
**Fixes**
This commit fixes https://github.com/pytorch/pytorch/issues/37668.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39116
Differential Revision: D22082806
Pulled By: SplitInfinity
fbshipit-source-id: 1227e243ab941376cd5e382fb093751e88dc8846
