Resolve relative import for modeldef

Summary:
There is an issue with the relative import in the `__init__` file of modeldef that causes tests on GitHub CI to fail.
Specifically, the `FBNetV2ModelArch` is not correctly populated.
The internal CI does not detect such failures because we use the buck build system.
This diff fixes it.

Pull Request resolved: https://github.com/facebookresearch/d2go/pull/547

Reviewed By: patricksnape

Differential Revision: D46177424

fbshipit-source-id: 06b23b9b221c990cd15a2debff6def8cfb99743b

d2go/modeling/modeldef/__init__.py

@@ -7,4 +7,4 @@ This is the centralized place to define modeldef for all projects under D2Go.
 """
 
 # @fb-only: from d2go.modeling.modeldef import fb  # isort:skip  # noqa 
-from d2go.modeling import modeldef  # noqa
+from d2go.modeling.modeldef import modeldef  # noqa