[chat] fix bugs and add unit tests

[cwher]

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[cwher]

There are still several errors. DO NOT merge until they are resolved.

• Gemini in tests/test_checkpoint
  This is fixed by [checkpointio] Sharded Optimizer Checkpoint for Gemini Plugin #4302.
• Zero2 in tests/test_checkpoint
• Zero2 + Lora in tests/test_train's stage 1 & 2