Add group quantization for whisper

[jimypbr]

What does this PR do?

Adds group quantization custom op to optimum. Adds the functionality to whisper's linear layers.

Still to do:

• Automate the custom op build into setup.py
• Add quantisation options into IPUConfig or whisper parallelize kwargs.

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

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