Upstream MultiTypeExperiment features into Experiment

[mpolson64]

Summary:
These changes will enable us to deprecate multitypeexperment, simplifying the Ax data model ahead of storage changes.

1. In Experiment make the default_trial_type a new Key.DEFAULT_TRIAL_TYPE value instead of None
2. Move over logic for bookkeeping metric -> trial_type and runner -> trial_type mappings
3. Treat LONG_ and SHORT_RUN trial types as special cases which map to DEFAULT_TRIAL_TYPE (i.e. if a Trial has trial_type=LONG_RUN then use whichever metrics and runners are mapped to DEFAULT_TRIAL_TYPE
4. Fix tests which expect the default_trial_type of an Experiment to be None

This diff allows us to remove all isinstance(foo, MultiTypeExperiment) checks in Ax in the next diff, then to deprecate MultiTypeExperiment entirely.

Differential Revision: D91618283

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.11215% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.73%. Comparing base (8812057) to head (73b1886).

Files with missing lines	Patch %	Lines
ax/core/experiment.py	74.62%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4873      +/-   ##
==========================================
- Coverage   96.76%   96.73%   -0.03%     
==========================================
  Files         591      591              
  Lines       61879    61949      +70     
==========================================
+ Hits        59875    59927      +52     
- Misses       2004     2022      +18     

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.25926% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.73%. Comparing base (8a706b8) to head (db3a3f2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ax/core/experiment.py	74.62%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4873      +/-   ##
==========================================
- Coverage   96.76%   96.73%   -0.03%     
==========================================
  Files         591      591              
  Lines       61955    62025      +70     
==========================================
+ Hits        59948    60001      +53     
- Misses       2007     2024      +17     

☔ 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.

[meta-codesync]

@mpolson64 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D91618283.