`__getattr__` and pytype checking

Currently `nn.Module` implement `__getattr__`. This disable any type checking when making a typo in attribute:

```python
my_module = MyModule()

my_module.non_existing_attribute # << pytype do not detect this
```

It would be nice to wrap `__getattr__` in a `if not typing.TYPE_CHECKING:` so that pytype actually check the attribute types.

Note that `class Module` would have to be updated so that the `Module` fields are defined (note doing this today would also allow type checking on `module.scope`,...):

```python
class Module:
scope: Optional[Scope]
```