Model validation code is not called
🐛 Bug
My defined methods for validation_step as well as validation_epoch_end do not seem to get called.
To Reproduce
Just call the provided code sample. Python should show the NotImplementedError. Instead the model completes 'successfully'.
Code sample
import pytorch_lightning as pl
import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.utils.data import DataLoader


class Dataset(torch.utils.data.IterableDataset):
    def __init__(self):
        super().__init__()

    def __iter__(self):
        def get_sample():
            for _ in range(5):
                yield torch.randn(20)
        return get_sample()

    def __len__(self):
        return 5

class Model(pl.LightningModule):
    def __init__(self):
        super().__init__()

        self.enc = nn.Linear(20, 10)
        self.dec = nn.Linear(10, 20)

    def forward(self, x):
        x = self.enc(x)
        x = F.relu(x)
        x = self.dec(x)
        return x

    def training_step(self, batch, batchIdx):
        x = self.forward(batch)
        return {'loss': torch.mean(x)}

    def validation_step(self, batch, batchIdx):
        raise NotImplementedError()
        x = self.forward(batch)
        return {'val_loss': torch.mean(x)}

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

    def configure_optimizers(self):
        return torch.optim.AdamW(self.parameters())

if __name__ == '__main__':
    trainer = pl.Trainer(num_sanity_val_steps=0)

    net = Model()
    dataset = Dataset()

    trainer.fit(net, train_dataloader=DataLoader(dataset, batch_size=8, num_workers=0), val_dataloaders=DataLoader(dataset, batch_size=8, num_workers=0))
Expected behavior
Instead, I'd expect the code sample above to fail.
Environment
Collecting environment information...
PyTorch version: 1.5.1+cu101
Is debug build: No
CUDA used to build PyTorch: 10.1

OS: Gentoo Base System release 2.7
GCC version: (Gentoo 9.3.0 p1) 9.3.0
CMake version: version 3.17.3

Python version: 3.6
Is CUDA available: Yes
CUDA runtime version: 10.1.243
GPU models and configuration: GPU 0: GeForce GT 730
Nvidia driver version: 440.82
cuDNN version: /opt/cuda/targets/x86_64-linux/lib/libcudnn.so.7.6.5

Versions of relevant libraries:
[pip3] numpy==1.19.0
[pip3] pytorch-lightning==0.8.1
[pip3] torch==1.5.1+cu101
[pip3] torchvision==0.6.1+cu101
[conda] Could not collect