C++ CustomClass in Python: indicate which methods are not implemented (#100171)
Without these changes, it can be hard to know which magic methods are not implemented on a given ScriptObject.
before:
```py
torch.ops.load_library("somelib.so")
c = torch.classes.somelib.SomeClass()
print(len(c))
# raise NotImplementedError
```
after:
```py
torch.ops.load_library("somelib.so")
c = torch.classes.somelib.SomeClass()
print(len(c))
# raise NotImplementedError: '__len__' is not implemented for __torch__.torch.classes.somelib.SomeClass
```
------
I could not find a linked issue, if you want me to open one as well I can do this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100171
Approved by: https://github.com/ezyang
