Added description of quantization_config
Author: vamsivallepu
Pull request: yes

Added missing description about quantization_config in replace_with_bnb_linear for better readability.

# What does this PR do?

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Fixes #31132 (issue)


## Before submitting
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- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
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- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
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Comments:
@younesbelkada @SunMarc 

Can you run the styling checks? `make fixup`

@younesbelkada  @SunMarc all tests passed now. Can we merge this now?

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