Fix the logging of a nested dictionary metric in MLflow

[KumoLiu]

Fix https://github.com/Project-MONAI/model-zoo/issues/697

Description

Flatten the metric dict when the metric is a nested dictionary.

Types of changes

• [x] 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.

[KumoLiu]

/build