Fixes #.

Description

• When the image is large, the number of instances may exceed np.uint8, so the default dtype should set to np.int64
• a workaround way to add FillHoles before generate_instance_contour, may prevent the issue when instances contain hole

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.