misc: make m8d-send-recv script flexible

[myungjin]

Description

This code change allows to specify rank 0 flexibly, meaning that a process belonging to more than one world can be a worker in one world and a leader in another world.

Type of Change

• [ ] Bug Fix
• [x] New Feature
• [ ] Breaking Change
• [ ] Refactor
• [ ] Documentation
• [ ] Other (please describe)

Checklist

• [ ] I have read the contributing guidelines
• [ ] Existing issues have been referenced (where applicable)
• [ ] I have verified this change is not present in other open pull requests
• [ ] Functionality is documented
• [ ] All code style checks pass
• [ ] New code contribution is covered by automated tests
• [ ] All new and existing tests pass