[Dynamo] Add unit test for explicitly calling __call__ (#100146)
@wconstab As we discussed last Friday, I added the unit test for explicitly calling __call__ and added comment to explain why we redirecting ```UserMethodVariable.call_function``` to ```NNModuleVariable.call_method``` for a certain case.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100146
Approved by: https://github.com/wconstab
