Add type hint stub (#780)

Summary:
Fixes #779

### Changes

- Add `py.typed` file stub
- Include the file into package data

Pull Request resolved: #780

Reviewed By: NivekT

Differential Revision: D39620230

Pulled By: ejguan

fbshipit-source-id: 0661c08bfdaad66a5e50506937d1339fb2865fec

Author: ejguan

setup.py

@@ -182,6 +182,7 @@ def remove_extension(pattern):
         ],
         package_data={
             "torchdata": [
+                "py.typed",
                 "datapipes/iter/*.pyi",
                 "datapipes/map/*.pyi",
             ],