Exclude 'step' from Sensitivity analysis by fixing step at full-progression 'step'

[sunnyshen321]

Summary:
This diff excludes the "step" feature (MAP_KEY) from sensitivity analysis plots by adding exclude_map_key and target_step parameters throughout the sensitivity computation. When exclude_map_key=True (the new default), the ax_parameter_sens() function fixes the step feature at the specified target_step value (or the maximum step if not provided) using a FixedFeatureModel wrapper, then computes Sobol indices over the remaining parameters.

This ensures "step" never appears in the output, making sensitivity plots more interpretable for users who care about parameter effects at final performance rather than across all training steps.

Differential Revision: D90819468

[meta-codesync]

@sunnyshen321 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D90819468.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 48.05195% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.61%. Comparing base (3156389) to head (29b8e17).

Files with missing lines	Patch %	Lines
ax/utils/sensitivity/sobol_measures.py	29.72%	26 Missing ⚠️
ax/plot/feature_importances.py	54.34%	21 Missing ⚠️
ax/utils/sensitivity/fixed_feature_model.py	45.45%	18 Missing ⚠️
ax/utils/sensitivity/derivative_measures.py	16.66%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4777      +/-   ##
==========================================
- Coverage   96.73%   96.61%   -0.12%     
==========================================
  Files         585      586       +1     
  Lines       61196    61326     +130     
==========================================
+ Hits        59195    59251      +56     
- Misses       2001     2075      +74     

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