Visualize Histogram metrics in Wandb

[grananqvist]

pfl supports reporting metrics to Weights & biases with https://apple.github.io/pfl-research/reference/callback.html#pfl.callback.WandbCallback.
Currently we only report the line metrics printed out in console.
Wandb has support for visualizing histograms.
It can be very useful if we can make use of this feature to visualize histogram of a metric over the cohort of users.
This depends on having #91 implemented such that we can use that data for wandb visualization.