Log feature density histograms to weights and biases

[jbloomAus]

See here

To do:

• [ ] Consider the best way to calculate these
• [ ] Log these to wandb (either via wandb histogram or plotly)
• [ ] Also log number of dead features

There's a few technical details here that could matter but any solution here would be a good start.

[HoagyC]

This should be closed, histograms and number of dead features are now solidly in wandb.