`LightningModule.log(..., on_epoch=True)` logs with `global_step` instead of `current_epoch`
🐛 Bug
When logging using:
self.log(f"some_metric", value, on_step=False, on_epoch=True)
in ie. training_step, the data is logged to the tensorboard with X axis in steps instead of epochs:

Expected behavior is for the x axis to be in epochs:

To Reproduce
(I'll try to work on reproduction example once I find some free cycles this week)
Environment
Pytorch 1.7 and Lightning 1.1rc
Additional context
@tchaton