[Observer Restructure]: Separate out scale/zp and observer init; separate out calibration from forward pass

[dsikka]

Summary

• Main goal of this PR is to separate out observer functionality which has been intertwined with other steps within compressed-tensors, specifically calibration/forward pass
• The purpose of this PR is to work as a stepping stone PR to move the observer code from compressed-tensors
• These changes include:

1. Separate out the observer and zp/scale initialization; scale and zp initialization will remain as is while observer initialization will move out of compressed-tensors
2. Separate calibration from the forward pass

Testing:

• Tested end-to-end
• Will need to update function call in llm-compressor, handled by Fix forward function pass call vllm-project/llm-compressor#845

[markurtz]

@dsikka double checking, this is ready for review now?

[dsikka]

@dsikka double checking, this is ready for review now?

Yup

[kylesayrs]

So much clearer

[rahul-tuli]

LGTM! with minor nits