Add num_trials trace to BenchmarkResult

[saitcakmak]

Summary:
The optimization_trace and other traces on BenchmarkResult are indexed by "completion event" rather than trial number. In the asynchronous case, multiple trials can complete at the same simulated time and get grouped into a single trace entry, so the trace length can be less than the number of trials. Previously there was no way to determine how many trials had completed at each trace entry.

This diff adds a num_trials field to BenchmarkResult (a list[int]) that records the cumulative number of completed or early-stopped trials at each completion event. In the synchronous case this is simply [1, 2, ..., n]. In the async case it can increase by more than 1 at a step, e.g. [2, 4, 5, ...]. The field is optional (None) for backwards compatibility with old stored results.

On AggregatedBenchmarkResult, the mean num_trials across replications is added as a new column on the optimization_trace and score_trace DataFrames (when available on all results).

Reviewed By: hvarfner

Differential Revision: D93751894

[meta-codesync]

@saitcakmak has exported this pull request. If you are a Meta employee, you can view the originating Diff in D93751894.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.80%. Comparing base (052a554) to head (975f98f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4941   +/-   ##
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  Coverage   96.80%   96.80%           
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  Files         595      595           
  Lines       63193    63213   +20     
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+ Hits        61173    61194   +21     
+ Misses       2020     2019    -1     

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