Phase 2: Migrate to StrEnum

[Balandat]

Summary:
Convert (str, Enum) and Enum patterns to Python 3.11's built-in StrEnum class.
This removes the need for the dual inheritance pattern and removes pyre-fixme
annotations that were needed to annotate enum values with str type.

Files changed:

• ax/utils/common/constants.py - Keys(str, Enum) -> Keys(StrEnum)
• ax/storage/utils.py - MetricIntent(enum.Enum) -> MetricIntent(StrEnum)
• ax/core/auxiliary.py - AuxiliaryExperimentPurpose(Enum) -> AuxiliaryExperimentPurpose(StrEnum)
• ax/utils/stats/model_fit_stats.py - ModelFitMetricDirection(Enum) -> ModelFitMetricDirection(StrEnum)
• ax/metrics/chemistry.py - ChemistryProblemType(Enum) -> ChemistryProblemType(StrEnum)
• ax/metrics/sklearn.py - SklearnModelType(Enum), SklearnDataset(Enum) -> StrEnum

Differential Revision: D91648882

[meta-codesync]

@Balandat has exported this pull request. If you are a Meta employee, you can view the originating Diff in D91648882.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.75%. Comparing base (4761470) to head (79166e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4868   +/-   ##
=======================================
  Coverage   96.75%   96.75%           
=======================================
  Files         589      589           
  Lines       61835    61836    +1     
=======================================
+ Hits        59830    59831    +1     
  Misses       2005     2005           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.