Refer to "time interval" to "step" in benchmarking; give `BenchmarkTestFunction` an "step" attribute

[esantorella]

Summary: I find this clearer, since "time interval" is vauge and could in theory refer to many different periods of time. The downside is that "epoch" may make less sense outside the context of AutoML.

The new attribute is not needed yet, but will be in the next diff.

Differential Revision: D66397779

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This pull request was exported from Phabricator. Differential Revision: D66397779

[codecov-commenter]

Codecov Report

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

Project coverage is 95.43%. Comparing base (7f2c4a0) to head (1e991b5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3110 +/- ## ======================================= Coverage 95.43% 95.43% ======================================= Files 503 503 Lines 50264 50267 +3 ======================================= + Hits 47967 47970 +3 Misses 2297 2297 ```

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This pull request was exported from Phabricator. Differential Revision: D66397779

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66397779

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66397779

[facebook-github-bot]

This pull request has been merged in facebook/Ax@3327deeab2a8b0cd6b7e15ecdd8a3d527fc863f2.