Disabling validation with val_percent_check=0.0 not working

[thegyro]

🐛 Bug

I am trying to train on a small subset of data by settting train_percent_check and val_percent_check flags. I am not able to disable validation by setting ver_percent_check=0.0. Despite it being set to 0, validation_epoch_end is executed.

To Reproduce

Steps to reproduce the behavior: Running the MNIST model on the Google Colab link

Code sample

mnist_model = MNISTModel()

# most basic trainer, uses good defaults (1 gpu)
trainer = pl.Trainer(gpus=1, val_percent_check=0.0, train_percent_check=0.1)    
trainer.fit(mnist_model)

Expected behavior

I expected the validation_epoch_end to not be invoked but it is.

Environment

Running on Google Colab with the sample MNIST tutorial

[github-actions]

Hi! thanks for your contribution!, great first issue!

[awaelchli]

I can confirm there is an issue. I tested it with the pl_example gpu template on master. I get a different error:

...
  File "C:\Users\aeduw\Documents\Repositories\pytorch-lightning\pytorch_lightning\trainer\training_loop.py", line 437, in run_training_epoch
    self.run_evaluation(test_mode=self.testing)
  File "C:\Users\aeduw\Documents\Repositories\pytorch-lightning\pytorch_lightning\trainer\evaluation_loop.py", line 368, in run_evaluation
    eval_results = self.evaluate(self.model, dataloaders, max_batches, test_mode)
  File "C:\Users\aeduw\Documents\Repositories\pytorch-lightning\pytorch_lightning\trainer\evaluation_loop.py", line 312, in evaluate
    eval_results = model.validation_epoch_end(outputs)
  File "C:\Users\aeduw\Documents\Repositories\pytorch-lightning\pl_examples\basic_examples\lightning_module_template.py", line 190, in validation_epoch_end
    val_loss_mean /= len(outputs)
ZeroDivisionError: division by zero

I think I know how to fix it and will submit a PR soon + unit test.

[awaelchli]

@thegyro This should now be fixed. I tested it in MNIST colab by installing PL from master branch:
! pip install git+https://github.com/PytorchLightning/pytorch-lightning.git@master --upgrade
Feel free to reopen if there are unresolved issues.