When setting lora_bias=True, a bias term is added to lora_B (huggingface#2237).
However, to merge this LoRA adapter, we need the base layer to also have
a bias. This has not been checked so far.

With this PR, we will now warn the user when we detect this situation.
Thus they can decide if they want to continue with this setting or not.
If they don't intend to merge, they're fine.

On top of this, when trying to merge in this situation, we now raise an
appropriate error that clearly explains why merging failed.

About PeftWarning

This PR adds a new warning class, PeftWarning. This makes it easier for
users to add PEFT specific warning filters (say, to ignore them or to
raise an error).

There are many more warnings in PEFT that could be migrated to this new
warning class (or a subclass where appropriate). This is outside the
scope of this PR.

Alternatives

1. We considered raising an error instead of warning when encountering
said situation. Many users miss warnings, so an error would be a
stronger signal. This would, however, be too harsh, as it could break
existing user code that is working perfectly fine.

2. We considered adding a bias term to the base layer when it is missing
during the merge. However, this requires careful bookkeeping (e.g. when
unmerging all adapters, the bias needs to be removed again). Moreover,
when calling merge_and_unload, users expect the original model
architecture to be returned. Suddenly adding a bias term would be
unexpected and could lead to errors down the line.