Signed-off-by: Can Zhao [email protected]

Fixes #4582 .

Description

correct a bug in convert_mask_to_box, will fail for multi-class detection

Status

Ready/Work in progress/Hold

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• 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.