assume empty lora dropout means 0.0 and add tests (#2243)

* assume empty lora dropout means 0.0 and add tests

* remove un-necessary arg

* refactor based on pr feedback:

* chore: lint

src/axolotl/utils/config/models/input/v0_4_1/__init__.py

@@ -367,6 +367,13 @@ class LoraConfig(BaseModel):
             loraplus_lr_embedding = float(loraplus_lr_embedding)
         return loraplus_lr_embedding
 
+    @model_validator(mode="before")
+    @classmethod
+    def validate_lora_dropout(cls, data):
+        if data.get("adapter") is not None and data.get("lora_dropout") is None:
+            data["lora_dropout"] = 0.0
+        return data
+
 
 class ReLoRAConfig(BaseModel):
     """ReLoRA configuration subset"""