TypeError: validation_step() takes 3 positional arguments but 4 were given
🐛 Bug

When running my model I get the error message: TypeError: validation_step() takes 3 positional arguments but 4 were given
Stacktrace:
line 106, in <module> trainer.fit(model)
line 707, in fit self.run_pretrain_routine(model)
line 812, in run_pretrain_routine self.evaluate(model, self.get_val_dataloaders(),self.num_sanity_val_steps, self.testing)
line 234, in evaluate
    test)
line 365, in evaluation_forward
    output = model.validation_step(*args)

To Reproduce
Steps to reproduce the behavior:

Install PyTorch-lightning with pip install pytorch-lightning
Follow the tutorial from William Falcon
Run it
See error


Code sample

Here is my validation step:
    def validation_step(self, val_batch, batch_idx):
        x, y = val_batch
        logits = self.forward(x)
        loss = self.cross_entropy_loss(logits, y)
        return {'val_loss': loss}

Expected behavior

No error
Environment
Python version: 3.7
Is CUDA available: Yes
CUDA runtime version: 10.2.89
GPU models and configuration: GPU 0: GeForce GTX 1050 Ti
Nvidia driver version: 442.19
cuDNN version: Could not collect
Versions of relevant libraries:
[pip3] numpy==1.16.1
[conda] blas                      1.0                         mkl
[conda] mkl                       2020.0                      166
[conda] mkl-service               2.3.0            py37hb782905_0
[conda] mkl_fft                   1.0.15           py37h14836fe_0
[conda] mkl_random                1.1.0            py37h675688f_0
[conda] pytorch                   1.4.0           py3.7_cuda101_cudnn7_0    pytorch
[conda] pytorch-ignite            0.4.0.dev20200229          pypi_0    pypi
[conda] pytorch-lightning         0.6.0                    pypi_0    pypi
[conda] torchvision               0.4.1                    pypi_0    pypi
[conda] torchviz                  0.0.1                    pypi_0    pypi
Additional context

Related Issue I think:
#105