Allow mask prompt in config (#1314)

llms/mlx_lm/lora.py

@@ -62,6 +62,7 @@ CONFIG_DEFAULTS = {
     "grad_checkpoint": False,
     "lr_schedule": None,
     "lora_parameters": {"rank": 8, "alpha": 16, "dropout": 0.0, "scale": 10.0},
+    "mask_prompt": False,
 }
 
 
@@ -99,7 +100,7 @@ def build_parser():
         "--mask-prompt",
         action="store_true",
         help="Mask the prompt in the loss when training",
-        default=False,
+        default=None,
     )
 
     parser.add_argument(