Address doc comments in TrainingLoop Callbacks

[xihui-wu]

This change addressed the doc comments per requested in #668.

Two questions from there remains for further discussion:

1. Use CSVLogger or CSVLog.
2. ProgressPrinter does not need to exist as a type but:
   extension TrainingLoopProtocol {
   public mutating func printProgress(event: TrainingLoopEvent) throws { ... }
   }

[dabrahams]

Some feedback; GTG now but can give more later.

[dabrahams]

I still don't know what a handler is. Still don't think this is a strong abstraction.

[xihui-wu]

It's an observer callback where TrainingLoop is designed upon. https://docs.google.com/document/d/1CtVFhV8OcQ4E7CmNyfFeZu0IgnUPx86tfQGXiHJUXz0/edit?ts=5ebef977#heading=h.b2so9ayrnyyp

You proposed to make it a function in TrainingLoop. Here are some points I'm more in favor of making callbacks wrapped in a separate classes:

1. Decouple from TrainingLoop
2. Use stored properties to share callback settings
3. Follow this pattern for all callbacks

Let's discuss more offline or in seminar meeting !?!

[dabrahams]

Suggested change

	/// Column names include "epoch", "batch" and `stats` `name`s.
	/// Column names are "epoch", "batch" and the `name` of each element of `stats`, in that order.

[dabrahams]

Actually the ordering of dictionary elements, for a given dictionary value (its contents) is not deterministic and if this component isn't used in exactly the way you've tested it, it will be printing the wrong stats under a given column, or printing a column that contains statistic A in one row but statistic B in a second row.

I would accept an array of statistic names in the init() of this component, which determines the column order.

[xihui-wu]

The order will preserve. I made stats to be an array of tuples instead of dictionary.

[dabrahams]

LGTM!