Allow pending trials to complete when failure rate is exceeded

[SebastianAment]

Summary: This commit allows any pending trials to be completed when the maximum permissible failure rate is exceeded. This is achieved adding a private Boolean attribute _failure_rate_has_been_exceeded, which causes _get_max_pending_trials to return returning 0 when it is true, leading the scheduler to stop adding new trials.

Differential Revision: D65943818

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This pull request was exported from Phabricator. Differential Revision: D65943818

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.69%. Comparing base (7b671dc) to head (31be013).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3070 +/- ## ========================================== - Coverage 95.69% 95.69% -0.01% ========================================== Files 488 488 Lines 49510 49526 +16 ========================================== + Hits 47381 47396 +15 - Misses 2129 2130 +1 ```

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D65943818

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D65943818

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D65943818

[facebook-github-bot]

This pull request has been merged in facebook/Ax@fa4c3b5b4f7601d3bae9a3686f47d8f6c04b576d.