Make tests easier to debug

[gdalle]

• Turn the InferOptTestUtils submodule into an importable package
• Change dispatch model of test_pipeline
• Make the Flux gradient computation and update more explicit

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (bb37b1a) 85.42% compared to head (0039d40) 85.42%. Report is 1 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #91 +/- ## ======================================= Coverage 85.42% 85.42% ======================================= Files 19 19 Lines 391 391 ======================================= Hits 334 334 Misses 57 57 ```

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