Fix push_to_hub by not calling create_branch if PR branch
Fix push_to_hub by not calling create_branch if PR branch (e.g. `refs/pr/1`).

Note that currently create_branch raises a 400 Bad Request error if the user passes a PR branch (e.g. `refs/pr/1`).

EDIT:
~~Fix push_to_hub by not calling create_branch if branch exists.~~

Note that currently create_branch raises a 403 Forbidden error even if all these conditions are met:
- exist_ok is passed
- the branch already exists
- the user does not have WRITE permission

Fix #7067.

Related issue:
- https://github.com/huggingface/huggingface_hub/issues/2419