Break apart session processor and the running of each session into se…

[brandonrising]

…parate classes

Summary

Breaks up the currently scary to update session processor into separate components and provides callback function passthroughs which allow you to define tasks be run before/after sessions, as well as before/after each node.

QA Instructions

Run invocations of various types and batch sizes. Compare output images of same inputs against those generated on current main. Validate logging/stats are formatted correctly. Intentionally run malformed graphs and nodes which will error out to ensure error reporting is still correct.

Merge Plan

I will merge once the PR has been thoroughly reviewed, tested, and approved on multiple OS/environments.

Checklist

• [ ] The PR has a short but descriptive title, suitable for a changelog
• [ ] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)