Tensorboard display nothing
🐛 Bug

To Reproduce
Steps to reproduce the behavior:
I run the code below successfuly, and when i open my tensorboard, scalar display in board, but there's not value on the plot, vert confusing~

Code sample

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

        # network
       
        # others
        self.criterion = MSELoss()
        self.logger_kun = init_logger("dnn", notebook=True)
        

    def forward(self, x):
        x = self.model.forward(x)
        return x
    
    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters())
    
    def training_step(self, batch, batch_idx):
        x, y, data_load_times = batch
        y_pred = self(x)
        
        loss = self.criterion(y_pred.squeeze(), y)
        self.logger.experiment.add_scalar('loss', loss, self.global_step)
        result = pl.TrainResult(minimize=loss)
        result.log("train_loss", loss, on_epoch=True)
        return result
        
    def validation_step(self, batch, batch_idx):
        x, y, data_load_times = batch
        y_pred = self(x)
        loss = self.criterion(y_pred.squeeze(), y)
        
        result = pl.EvalResult(checkpoint_on=loss)
        result.log("valid_loss", loss)
        return result

Expected behavior

Display value on tensorboard
Environment
Please copy and paste the output from our
environment collection script
(or fill out the checklist below manually).
You can get the script and run it with:
wget https://raw.githubusercontent.com/PyTorchLightning/pytorch-lightning/master/tests/collect_env_details.py
# For security purposes, please check the contents of collect_env_details.py before running it.
python collect_env_details.py


CUDA:

GPU:
available:         False
version:           10.2


Packages:

numpy:             1.18.1
pyTorch_debug:     False
pyTorch_version:   1.6.0
pytorch-lightning: 0.9.0
tensorboard:       2.2.0
tqdm:              4.42.1


System:

OS:                Linux
architecture:

64bit



processor:         x86_64
python:            3.7.6
version:           #47~18.04.1-Ubuntu SMP Thu May 7 13:10:50 UTC 2020



Additional context