More flexible paths for saving tensorboard data
🚀 Feature

I'd like more flexibility in the directory hierarchy used for writing tensorboard data.
Specifically, I want to make the experiment-name directory optional, removing one level in the hierarchy.
Also I'd like to be able to specify the name of the version directory.  Currently it is "version_N" but if the constructor version parameter is a string, I think it could just directly use that.
Motivation
You could specify the version on the command line or generate it from the current datetime, which I find more meaningful than "version_73".  Likewise if the tensorboard data is not going to be preserved, it's much more readable in the tensorboard gui to skip the experiment name.
Pitch
I have a branch with working code + tests
https://github.com/bobkemp/pytorch-lightning/tree/tensorboard-path-generalisation
Alternatives
None
Additional context