remove export_predictor customization from facegen

Summary: Customization via `export_predictor` is now deprecated, this diff move the functionality to `FacegenExportMethod`.

Reviewed By: danthe3rd

Differential Revision: D27935492

fbshipit-source-id: 4f35cff7f3709eff290edefce570cfeea47e687d

d2go/export/api.py

@@ -203,7 +203,7 @@ def default_export_predictor(
     model_inputs = (
         export_config.data_generator(inputs)
         if export_config.data_generator is not None
-        else None
+        else (inputs,)
     )
 
     predictor_path = os.path.join(output_dir, predictor_type)