Validation Loss: Metric not available
🐛 Bug
The metric val_loss was not found for ReduceLROnPlateau and progress bar display. But using print(val_loss on validation_step, and validation_epoch_end works ok (display Tensor(value).
Code sample
class MyModel(pl.LightningModule):    
    def __init__(self, train_df, val_df, test_df, hparams = Namespace(lr = 0.02)):
        # Initialization
        super(MyModel, self).__init__()
        self.train_df = train_df
        self.val_df = val_df
        self.test_df = test_df
        self.hparams = hparams
        
        # Model Structure
        backbone = models.resnet18(pretrained=False)
        self.features_extractor = torch.nn.Sequential(*list(backbone.children())[:-1])
        self.fc = torch.nn.Sequential(*[
            torch.nn.Linear(backbone.fc.in_features, 256, bias=True),
            torch.nn.Linear(256, 32, bias=True),
            torch.nn.Linear(32, 4, bias=True)
        ])
        
        # Loss
        self._loss = torch.nn.CrossEntropyLoss(weight=weight.float())
    
    def forward(self, x):
        x = self.features_extractor(x)
        x = x.squeeze(-1).squeeze(-1)
        x = self.fc(x)
        return x
    
    def loss(self, logits, y):
        return self._loss(logits, y)
    
    def training_step(self, batch, batch_idx):
        # 1. Inference
        x, y = batch
        y_hat = self.forward(x)
        
        # 2. Loss
        loss = self.loss(y_hat, y)
        
        # 3. Output
        tensorboard_logs = {'train_loss': loss}
        return {'loss': loss, 'log': tensorboard_logs}

    def validation_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self.forward(x)
        loss = self.loss(y_hat, y)
        return {'val_loss': loss}

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

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=self.hparams.lr, weight_decay=0.01)
        scheduler = torch.optim.lr_scheduler.ReduceLROnPlateau(optimizer)
        return [optimizer], [scheduler]

    def prepare_data(self):
        self.train_ds = ClassificationDataset(self.train_df, 'data/images')
        self.val_ds = ClassificationDataset(self.val_df, 'data/images')

    def train_dataloader(self):
        return torch.utils.data.DataLoader(self.train_ds, batch_size=256, num_workers=4, sampler=train_sampler)

    def val_dataloader(self):
        return torch.utils.data.DataLoader(self.val_ds, batch_size=64, num_workers=4)
    
Error
model = MyModel(train_df, val_df, test_df, hparams=Namespace(lr=0.001))
trainer = pl.Trainer(gpus=1, max_epochs=2, train_percent_check=0.01, weights_summary='top')
trainer.fit(model)
---------------------------------------------------------------------------
MisconfigurationException                 Traceback (most recent call last)
<ipython-input-412-55f3b29fc11e> in <module>
      4 # Trainer
      5 trainer = pl.Trainer(gpus=1, max_epochs=2, train_percent_check=0.01, weights_summary='top')
----> 6 trainer.fit(model)

/opt/conda/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py in fit(self, model, train_dataloader, val_dataloaders, test_dataloaders)
    702 
    703         elif self.single_gpu:
--> 704             self.single_gpu_train(model)
    705 
    706         elif self.use_tpu:  # pragma: no-cover

/opt/conda/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py in single_gpu_train(self, model)
    475             self.optimizers = optimizers
    476 
--> 477         self.run_pretrain_routine(model)
    478 
    479     def tpu_train(self, tpu_core_idx, model):

/opt/conda/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py in run_pretrain_routine(self, model)
    862 
    863         # CORE TRAINING LOOP
--> 864         self.train()
    865 
    866     def test(self, model: Optional[LightningModule] = None):

/opt/conda/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py in train(self)
    364 
    365                 # update LR schedulers
--> 366                 self.update_learning_rates(interval='epoch')
    367 
    368                 if self.max_steps and self.max_steps == self.global_step:

/opt/conda/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py in update_learning_rates(self, interval)
    779                         avail_metrics = ','.join(list(self.callback_metrics.keys()))
    780                         raise MisconfigurationException(
--> 781                             f'ReduceLROnPlateau conditioned on metric {monitor_key}'
    782                             f' which is not available. Available metrics are: {avail_metrics}.'
    783                             ' Condition can be set using `monitor` key in lr scheduler dict'

MisconfigurationException: ReduceLROnPlateau conditioned on metric val_loss which is not available. Available metrics are: . Condition can be set using `monitor` key in lr scheduler dict

Environment

CUDA:

GPU:

Tesla P100-PCIE-16GB


available:         True
version:           10.1


Packages:

numpy:             1.18.1
pyTorch_debug:     False
pyTorch_version:   1.4.0
pytorch-lightning: 0.7.3
tensorboard:       2.2.1
tqdm:              4.43.0


System:

OS:                Linux
architecture:

64bit



processor:
python:            3.7.6
version:           #1 SMP Debian 4.9.210-1 (2020-01-20)



Additional context
Dataset
class ClassificationDataset(torch.utils.data.Dataset):
    def __init__(self, df: pd.DataFrame, root_dir: pathlib.Path, test=False):
        self.df = df
        self.test = test
        self.root_dir = root_dir
        self.transforms = transforms.Compose([
            transforms.Resize(size=(224, 224)),
            transforms.ToTensor(),
            transforms.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])
        ])
        self.number_of_categories = len(self.df.time_cat.cat.categories)
    
    def __getitem__(self, index):
        if torch.is_tensor(index):
            index = index.tolist()
        sample = datasets.folder.default_loader(pathlib.Path(self.root_dir) / pathlib.Path(self.df.iloc[index]['filename']))
        sample = self.transforms(sample)
        y = int(self.df.time_cat.cat.codes.iloc[index])
        return (sample, y)
    
    def __len__(self):
        return self.df.shape[0]