added vllm fakequant export support

[kinjalpatel27]

What does this PR do ?

Added support to export quantized checkpoint from examples/post_training/modelopt/ for vLLM fakequant in modelopt/examples/vllm_serve/vllm_serve_fakequant.py

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[Phlip79]

Please fix conflicts.

[kinjalpatel27]

Please fix conflicts.

@Phlip79 I have fixed the conflicts. thank you

[kinjalpatel27]

@Phlip79 can you please review?

[dimapihtar]

LGTM. Thank you!

[dimapihtar]

/ok to test e0b17dc

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You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23501179852