[`bnb`] Force `requires_grad` to be `False`
# What does this PR do?

Addresses: https://github.com/huggingface/accelerate/pull/1237#discussion_r1146045485

Some users uses `replace_8bit_linear` as a standalone function, by default the `Linear8bitLinear` layers that are newly created have `requires_grad` set to `True` leading to a bug on `bitsandbytes` library. 

This PR forces `requires_grad` to be `False` for these layers to avoid these issues 

Related: https://github.com/huggingface/accelerate/pull/1237

All bnb slow tests pass

cc @sgugger 
_The documentation is not available anymore as the PR was closed or merged._