Add momentum to the LearningRateMonitor

[jeremyjordan]

🚀 Feature

We should add a flag to optionally log the momentum value (for optimizers that have the attribute) in our LearningRateMonitor.

Motivation

Some learning rate schedulers (eg. OneCycle) also adjust the momentum value when updating the learning rate.

Pitch

For each scheduler's optimizer, we can check to see if it has a momentum attribute. If so, we log the value in addition to the learning rate.

[jeremyjordan]

cc @rohitgr7 and @Borda - what do you think? I'm happy to open a PR for this.

[rohitgr7]

I like it 👍