[Profiler][Trivial] Move ID assignment code to `data_flow.cpp` (#87670)
ID assignment has become a very complex facet of the profiler. The existing code has grown organically as I've discovered various refinements and has become very difficult to understand or reason about. (With more complexity coming in https://github.com/pytorch/pytorch/pull/87133)
I want to take a step back and add some structure and additional comments to the ID assignment algorithm. Before I do, however, it's time to move it out of `collection.cpp` to a dedicated data flow file.
Differential Revision: [D40666360](https://our.internmc.facebook.com/intern/diff/D40666360/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D40666360/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87670
Approved by: https://github.com/slgong-fb
