Fixes #6417 .

Description

• Move the logics of creating ensemble output dir from ensemble to __init__ in EnsembleRunner to avoid processes competing with each other in dir creation
• Add checks in AutoRunner set_image_save_transform

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.