Use experiment._properties["total_concurrent_arms"] for determining n

[danielcohenlive]

Summary: In GS.gen_for_multiple_trials_with_multiple_models and GS.gen_with_multiple_nodes

Reviewed By: lena-kashtelyan

Differential Revision: D63422137

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This pull request was exported from Phabricator. Differential Revision: D63422137

[codecov-commenter]

Codecov Report

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

Project coverage is 95.68%. Comparing base (a144287) to head (d82f589).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2792 +/- ## ======================================= Coverage 95.68% 95.68% ======================================= Files 488 488 Lines 47985 48013 +28 ======================================= + Hits 45915 45943 +28 Misses 2070 2070 ```

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63422137

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63422137

[facebook-github-bot]

This pull request has been merged in facebook/Ax@b8c22426ad21157ac1c39f8eb7e54b06675916ca.