Validation loss in progress bar printed line by line
Common bugs:
checked.
Describe the bug
When adding a "progress_bar" key to the validation_end output, the progress bar doesn't behave as expected and prints one line per iteration, eg:
80%|8| 3014/3750 [00:23<00:01, 516.63it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
82%|8| 3066/3750 [00:23<00:01, 517.40it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
83%|8| 3118/3750 [00:23<00:01, 516.65it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 85%|8| 3170/3750 [00:23<00:01, 517.42it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
86%|8| 3222/3750 [00:23<00:01, 517.59it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_
87%|8| 3274/3750 [00:23<00:00, 518.00it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
89%|8| 3326/3750 [00:23<00:00, 518.16it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
90%|9| 3378/3750 [00:23<00:00, 518.45it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_
91%|9| 3430/3750 [00:23<00:00, 518.36it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
93%|9| 3482/3750 [00:23<00:00, 518.02it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
94%|9| 3534/3750 [00:24<00:00, 517.26it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
96%|9| 3586/3750 [00:24<00:00, 517.68it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
97%|9| 3638/3750 [00:24<00:00, 518.08it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_ 
98%|9| 3690/3750 [00:24<00:00, 518.18it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_
100%|9| 3742/3750 [00:24<00:00, 518.23it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_
100%|#| 3750/3750 [00:24<00:00, 518.23it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_loss=1.16]
save callback...
100%|#| 3750/3750 [00:24<00:00, 152.16it/s, batch_nb=1874, epoch=14, gpu=0, loss=1.070, training_loss=0.792, val_loss=1.16]

To Reproduce
Steps to reproduce the behavior:

Take MNIST script minimal example (https://williamfalcon.github.io/pytorch-lightning/LightningModule/RequiredTrainerInterface/)
with some code to run it

if __name__ == "__main__":
    model = CoolModel()

    # most basic trainer, uses good defaults
    default_save_path = '/tmp/checkpoints/'
    trainer = pl.Trainer(default_save_path=default_save_path,
                         show_progress_bar=True)
    trainer.fit(model)


Change validation_end method to:

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

        return {
                'progress_bar': tqdm_dict,
                'log': {'val_loss': avg_loss},
        }


Change training_step to:

    def training_step(self, batch, batch_nb):
        x, y = batch
        y_hat = self.forward(x)
        loss = F.cross_entropy(y_hat, y)
        output = {
            'loss': loss,  # required
            'progress_bar': {'training_loss': loss},  # optional (MUST ALL BE TENSORS)
        }
        return output


Run the script, see error at validation time.

Note that both steps 2 and 3 are necessary to reproduce the issue, each separately would run as expected.
Expected behavior
A progress bar on a single line.
Screenshots
If applicable, add screenshots to help explain your problem.
Desktop (please complete the following information):

OS: Linux
Version
pytorch-lightning==0.5.1.3
torch==1.2.0

Additional context
Actually I ran into this issue after trying to add EarlyStopping, which asked for val_loss, which I found out was to be added via the progress_bar metrics... which was quite unexpected for me (I would have had it in "log" or direct key?)