TypeError: validation_step() takes 3 positional arguments but 4 were given
Hi, I just started using lightning and I've been running into this bug lately.
Here's my code for the validation_step and validation_end:
def validation_step(self, batch, batch_idx):  
        imgs = batch
        (z1, z2) = torch.split(imgs['X'], 1, 1)
        ct = imgs['CT']
        y_hat = self.discriminator(self(z1, z2).detach())
        y = self.discriminator(ct.detach())
        print('Validation Step: \n')
        print('y_hat = ', y_hat)
        print('y_hat.shape = ', y_hat.shape)
        print('y = ', y)
        print('y.shape = ', y.shape)
        return {'val_loss': F.cross_entropy(y_hat, y)}

def validation_end(self, outputs):
       avg_loss = torch.stack([x['val_loss'] for x in outputs]).mean()
       return {'val_loss': avg_loss, 
                  'progress_bar': {avg_loss}}

Stacktrace:
Traceback (most recent call last):
  File "/home/students/lan/Schreibtisch/Code/x2ct_architecture_ff/gan.py", line 333, in <module>
    main(hparams)
  File "/home/students/lan/Schreibtisch/Code/x2ct_architecture_ff/gan.py", line 308, in main
    trainer.fit(model)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 692, in fit
    self.dp_train(model)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 471, in dp_train
    self.run_pretrain_routine(model)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 812, in run_pretrain_routine
    self.evaluate(model, self.get_val_dataloaders(), self.num_sanity_val_steps, self.testing)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/pytorch_lightning/trainer/evaluation_loop.py", line 234, in evaluate
    test)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/pytorch_lightning/trainer/evaluation_loop.py", line 349, in evaluation_forward
    output = model(*args)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/torch/nn/modules/module.py", line 541, in __call__
    result = self.forward(*input, **kwargs)
  File "/work/scratch/lan/conda/envs/pytorch_thesis/lib/python3.7/site-packages/pytorch_lightning/overrides/data_parallel.py", line 62, in forward
    return self.module.validation_step(*inputs[0], **kwargs[0])
TypeError: validation_step() takes 3 positional arguments but 4 were given

There has been a similar issue in the past, namely #105, but the bug was resolved in an earlier version and couldn't help me much.
Environment:

Ubuntu: 18.04.3 LTS
Python: 3.7.5
Pytorch: 1.3.1
Pytorch-Lightning: 0.6.0
Anaconda: 4.7.12

Thanks for the help!