Fix/warning msg rnn (#1674)

* warning

* warning

* adapt warning

---------

Co-authored-by: Dennis Bader <[email protected]>

Author: dumjax

darts/models/forecasting/rnn_model.py

@@ -380,6 +380,12 @@ def __init__(
         """
         # create copy of model parameters
         model_kwargs = {key: val for key, val in self.model_params.items()}
+
+        if model_kwargs.get("output_chunk_length") is not None:
+            logger.warning(
+                "ignoring user defined `output_chunk_length`. RNNModel uses a fixed `output_chunk_length=1`."
+            )
+
         model_kwargs["output_chunk_length"] = 1
 
         super().__init__(**self._extract_torch_model_params(**model_kwargs))