question: Issue in fitting model and finding optimal learning rate parameter
context: following is the error: NotImplementedError: `val_dataloader` must be implemented to be used with the Lightning Trainer
  
  LOCAL_RANK: 0 - CUDA_VISIBLE_DEVICES: [0]
  ---------------------------------------------------------------------------
  NotImplementedError                       Traceback (most recent call last)
  <ipython-input-11-263e8be26564> in <module>()
        3     tft,
        4     train_dataloader=train_dataloader,
  ----> 5     val_dataloaders=val_dataloader,
        6 )
  
  11 frames
  /usr/local/lib/python3.7/dist-packages/pytorch_lightning/core/hooks.py in val_dataloader(self)
      590             will have an argument ``dataloader_idx`` which matches the order here.
      591         """
  --> 592         raise NotImplementedError("`val_dataloader` must be implemented to be used with the Lightning Trainer")
      593 
      594     def predict_dataloader(self) -> EVAL_DATALOADERS:
  
  NotImplementedError: `val_dataloader` must be implemented to be used with the Lightning Trainer
  trainer.fit(
      tft,
      train_dataloader=train_dataloader,
      val_dataloaders=val_dataloader,
  )
answer: I think it's a problem with pytorch-lightning==1.5.0.
  Had this problem too with code that worked before recreating my venv and the difference was version 1.5.0 release on 2. Nov.
  Switched to 1.4.9 and it worked again.
  Checked 1.5.0rc1 and it did not work either.