Retry HfApi call inside push_to_hub when 504 error
Ass suggested by @lhoestq in #3872, this PR:
- Implements a retry function
- Retries HfApi call inside `push_to_hub` when 504 error. To be agreed:
  - max_retries = 2 (at 0.5 and 1 seconds)

Fix #3872.