PyText model exporter for SGNN (facebookresearch#464)

Summary:
Pull Request resolved: facebookresearch#464

- PyText exporter skeleton
- Caffe2 op for text Preprocessor

Reviewed By: ajaech

Differential Revision: D14894599

fbshipit-source-id: 3f33b4a0e894d197d1f37d4ffbc804fa958188cc

pytext/task/task.py

@@ -105,6 +105,7 @@ def from_config(cls, task_config, metadata=None, model_state=None):
                 task_config.labels,
                 data_handler.metadata,
                 task_config.model,
+                task_config.featurizer,
             )
             if task_config.exporter
             else None