refactor: restructuring packaging

[myungjin]

Description

Currently, world_manager.py and world_communicator.py are copied into torch package and they become part of torch after installing multiworld. This is inappropriate. We keep those files under multiworld package, but we patch pytorch such that the variables (_worlds, _World, etc) required by multiworld are exposed by patching init.py in distributed module. Therefore, pytorch patch file is updated and all other necessary changes are made to ensure that all the examples can be executed.

Type of Change

• [ ] Bug Fix
• [ ] New Feature
• [x] Breaking Change
• [x] 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