Update torch_forecasting_model.py

Corrected file saving process for checkpoint files (ckpt) to filter out occurrences of the string '.pt' from the previous file path."

Author: SaeedForoutan

darts/models/forecasting/torch_forecasting_model.py

@@ -1579,7 +1579,7 @@ def save(self, path: Optional[str] = None) -> None:
             torch.save(self, f_out)
 
         # save the LightningModule checkpoint
-        path_ptl_ckpt = path + ".ckpt"
+        path_ptl_ckpt = path.replace(".pt", "") + ".ckpt"
         if self.trainer is not None:
             self.trainer.save_checkpoint(path_ptl_ckpt)
         # TODO: keep track of PyTorch Lightning to see if they implement model checkpoint saving