[PyTorch] Add docstring for CP load balancing

[cyanguwa]

Description

This PR adds some docstring to explain the requirement of using TE-PyTorch CP. Namely, it assumes token re-ordering to achieve the DualChunkSwap loading balancing strategy.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Add docstring for CP load balancing.

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[cyanguwa]

#1780 contains the changes in this PR but would take longer to get to the finish line. Merging this to TE 2.4 first.

No GitLab CI needed as it's only docstring change - it doesn't go into TE docs. But more (and more proper) documentation on CP should be expected in the future.