support adam bf16 state

[XiaobingSuper]

Description

This PR supports Adam bf16 state to reduce memory usage, we have tested some smaller models or largest models (LLM models), and we get a similar convergence even with this bf16 lower precision state. For the deepseek-v3 report, they also used the bf16 state to reduce training memory usage.

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:

• Change A
• Change B

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

[XiaobingSuper]

@timmoon10 please help review it, thanks.