ENH error message when choosing wrong bias (#946)

Raise an error with a helpful error message when the user chooses an incorrect
option for the bias argument.

---------

Co-authored-by: datta0 <venkatadattasanimmaturi@gmail.com>

src/peft/tuners/lora/model.py

@@ -245,7 +245,7 @@ def _mark_only_adapters_as_trainable(self) -> None:
                 if isinstance(m, LoraLayer) and hasattr(m, "bias") and m.bias is not None:
                     m.bias.requires_grad = True
         else:
-            raise NotImplementedError
+            raise NotImplementedError(f"Requested bias: {bias}, is not implemented.")
 
     @staticmethod
     def _create_new_module(lora_config, adapter_name, target, **kwargs):