HfModel: Disable use_exllama by default for GPTQ models

[jambayk]

Describe your changes

The default value for use_exllama in transformers is True. However, exllama model cannot be loaded on cpu (for model export) and doesn't have a backward pass implemented for finetuning.
Since the main use for gptq quantized model in Olive is for export and finetuning, we should disable use_exllama by default. User can provide use_exllama=True as part of the loading args if they want to enable exllama for inference, etc.

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Lint and apply fixes to your code by running lintrunner -a
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.
• Is this PR including examples changes? If yes, please remember to update example documentation in a follow-up PR.

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