Create py.typed (#12893)

* Create py.typed

This creates a [py.typed as per PEP 561](https://www.python.org/dev/peps/pep-0561/#packaging-type-information) that should be distributed to mark that the package includes (inline) type annotations.

* Update setup.py

Include py.typed as package data

* Update setup.py

Call `setup(...)` with `zip_safe=False`.

setup.py

@@ -350,6 +350,8 @@ setup(
     url="https://github.com/huggingface/transformers",
     package_dir={"": "src"},
     packages=find_packages("src"),
+    package_data={"transformers": ["py.typed"]},
+    zip_safe=False,
     extras_require=extras,
     entry_points={"console_scripts": ["transformers-cli=transformers.commands.transformers_cli:main"]},
     python_requires=">=3.6.0",