[GSoC] Add Python 3 typing annotations to TFDS

Task: Add Python 3 typing annotations to TFDS repository. Now that Python 2 support is officially dropped, we can add better typing to TFDS.

Impact: Everyone using TFDS with a static type checker

Instructions:

• Add typing annotations to our API: (Typing annotation guide: https://docs.python.org/3/library/typing.html)
• Check that the typing annotations are valid with https://github.com/google/pytype
• Only update only a few files at a time (send multiple PR).
• As TFDS has many files, many people can work on this issue at the same time, just coordinate with each other.
• Updating the core/ is what matter the most. Then scripts/, Other files are optional
• Typing are ignored in test files, so no need to add typing annotations to *_test.py

Complexity: Easy

Start by selecting a small group of non-test files under core/, which the issue identifies as the priority; scripts/ is secondary. Read the typing guide and validate the annotations with pytype. Done means the selected files are annotated and pytype accepts them; do not add annotations to *_test.py files.