GitLab

This just adds a stub function that raises an assertion error; this
accomplishes two things:

- silences many pylint warnings
- if we ever stumble upon this, a specific assertion error is
  (hopefully) clearer than just a not implemented error
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>