[WIP] add mass feature to nx.kl_div and harmonize kl computation in the toolbox

[cedricvincentcuaz]

Types of changes

• Add feature mass=False|True in nx.kl_div to compute the generalized kl divergence either with the difference of total masses or not.
• Screened the toolbox to ensure proper use of nx.kl_div and along the way i set all existing offsets to nx.log to 1e-16.

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.67%. Comparing base (e530985) to head (368f979).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #654   +/-   ##
=======================================
  Coverage   96.66%   96.67%           
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  Files          85       85           
  Lines       16934    16956   +22     
=======================================
+ Hits        16370    16392   +22     
  Misses        564      564