Adding documentation for Kullback-leibler Divergence

[Shashi456]

Refer to this.

cc: @eaplatanios.

[rxwei]

Thanks for putting this together. A few comments:

1. Please wrap all math formulae within backquotes. In particular, p and q should be `p` and ` q`, respectively, and the formula on line 145 should also be backquoted.
2. Please use "and" instead of "&" in documentation comment sentences. "&" should be especially not be used in between math terms to avoid confusion with logical conjunction.
3. "KL-Distance" was not mentioned anywhere, and is very different from the standard terminology used in this API name. Naming and descriptive comments are very important to our APIs.
   • First, the short name should be "KL divergence", as it's the standard term used in the summary of this API and is a more canonical term of this operation.
   • Second, you should introduce the short name right after the first occurrence of the full name in a pair of parentheses. Namely, Returns the Kullback-Leibler divergence (KL divergence) between ....
4. In the current comment, there are two consecutive "between"s. Please make sure the sentence reads grammatically.
5. The formula on 145 appears to use an API called sum(_:), but such API does not exist in the our library and thus may not be relatable. Please use .sum() instead because that exists in our library. Also, for consistency, the / operator should leave a space before and after.
6. For conciseness, I would suggest

Given two distribution `p` and `q`, KL divergence computes `(p * log(p / q)).sum()`.

[Shashi456]

@rxwei thanks for the help, I've realized while writing doc comments is I tend to just consider if it makes sense to me and not someone who'd be interacting with the docs for the first time. I'll take more care from the next time

[rxwei]

Thank you!