Brain MMAR pretrained examples

[myron]

Adds example of pretrained MMAR models for brain tasks #2273

Status

**Ready

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.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[wyli]

thanks @myron, could you please sign the commit? https://github.com/Project-MONAI/MONAI/pull/2406/checks?check_run_id=2884207553