Second-order indices support in SobolSensitivity

[hvarfner]

Summary: Added support for second order Sobol indices in ax_parameter_sens. Old behavior is unchanged, but:

• Specifying order="second" gives first- and second-order indices in ax_parameter_sens
• SobolSensitivity outputs either first- or second-order indices depending on argument
• Second-order sensitivity requires first-order sensitivity to be computed
• Keys for second-order indices are a tuple ('param_a' , 'param_b')

Differential Revision: D63403218

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403218

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403218

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.68%. Comparing base (75b4bf8) to head (a81ae4b).

Files with missing lines	Patch %	Lines
ax/utils/sensitivity/sobol_measures.py	90.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2788 +/- ## ========================================== - Coverage 95.68% 95.68% -0.01% ========================================== Files 488 488 Lines 47978 48003 +25 ========================================== + Hits 45908 45931 +23 - Misses 2070 2072 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.