Ability to specify step for logged metrics
🚀 Feature
Add option to specify step for logged metrics
Motivation
After calculating some metric on n-th epoch I'd like to put corresponding mark (n) on x-axis. As I see in the source code, there's no obvious way to do this. Instead something like n * num_batches will be shown, like in the figure where the loss is calculated after each epoch.

Pitch
I'd love to see 0,1,2,... ticks on x-axis.
My proposal is to allow user to add step key in log dict, if this key is presented, corresponding value would be used in x-axis. I can make a PR if you agree with this idea.
Alternatives
It would be interesting to know how this could be achieved in other way.