[PEFT] Set eval mode when loading PEFT adapter (#34509)
* [PEFT] Set eval mode when loading PEFT adapter
Resolves #34469
When calling model.load_adapter to load a PEFT adapter, by default the
adapter should be set to eval mode. This is now correctly done. Users
can still pass is_trainable=True to load the adapter in training mode.
* Linter
