TensorBoardLogger not saving hparams without metrics
🐛 Bug
log_hyperparams for TensorBoardLogger saves no data with default metrics=None, only hparam entries/names show up in sidebar
To Reproduce
Steps to reproduce the behavior:
import pytorch_lightning as pl
logger = pl.loggers.TensorBoardLogger("./test_logs")
test_dict = {"test":0}
logger.log_hyperparams(test_dict)                      ## no data saved
logger.log_hyperparams(test_dict, test_dict)           ## works
logger.log_metrics(test_dict)                          ## works
logger.experiment.add_hparams(test_dict, test_dict)    ## works but saves in a different events file


Expected behavior
hparams data is saved and viewable via tensorboard with default args
Proposed solution
For default hparams logging without metrics, add a placeholder metric? I can do a PR if this is appropriate.