Rename brevitas quant custom op

[jinchen62]

I have tested the op would be torch.operator "quant.matmul_rhs_group_quant".

[jinchen62]

Pls review, thanks! @Giuseppe5 @volcacius @ harish

[Giuseppe5]

Would you be able to run
pre-commit run --all-files

And add also the changes that cause the pre-commit check to fail?
It should be changing something in one of the files you are already modifying.

Thanks!