Split out building stage instance in factories

[eacharles]

Problem & Solution Description (including issue #)

Code Quality

• [ ] My code follows the code style of this project
• [ ] I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• [ ] My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• [ ] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.40%. Comparing base (aa7a075) to head (7464fba).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #145 +/- ## ======================================= Coverage 98.39% 98.40% ======================================= Files 44 44 Lines 2370 2376 +6 ======================================= + Hits 2332 2338 +6 Misses 38 38 ```

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