Zero mAP warning on YAML val (#16698)

ultralytics/engine/validator.py

@@ -119,6 +119,8 @@ class BaseValidator:
             self.args.plots &= trainer.stopper.possible_stop or (trainer.epoch == trainer.epochs - 1)
             model.eval()
         else:
+            if Path(self.args.model).suffix == ".yaml":
+                LOGGER.warning("WARNING ⚠️ validating an untrained model YAML will result in 0 mAP.")
             callbacks.add_integration_callbacks(self)
             model = AutoBackend(
                 weights=model or self.args.model,