[JIT] Print better error when class attribute IValue conversion fails (#50255)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50255
**Summary**
TorchScript classes are copied attribute-by-attribute from a py::object into
a `jit::Object` in `toIValue`, which is called when copying objects from
Python into TorchScript. However, if an attribute of the class cannot be
converted, the error thrown is a standard pybind error that is hard to
act on.
This commit adds code to `toIValue` to convert each attribute to an
`IValue` inside a try-catch block, throwing a `cast_error` containing
the name of the attribute and the target type if the conversion fails.
**Test Plan**
This commit adds a unit test to `test_class_type.py`
based on the code in the issue that commit fixes.
**Fixes**
This commit fixes #46341.
Test Plan: Imported from OSS
Reviewed By: pbelevich, tugsbayasgalan
Differential Revision: D25854183
Pulled By: SplitInfinity
fbshipit-source-id: 69d6e49cce9144af4236b8639d8010a20b7030c0
