Checkpoint conversion between GPT_model and Hybrid_model

[guihong-nv]

What does this PR do ?

• Checkpoint conversion between GPT_model and Hybrid+model
• For GPT_model, we do not support the legacy models

Contribution process

Pre-checks

• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

Code review

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The proposed review process for `dev` branch is under active discussion.

MRs are mergable after one approval by either eharper@nvidia.com or zijiey@nvidia.com.

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[guihong-nv]

/ok to test 6278662