Add to regression log: flat indexes for MS MARCO v1 passage

[codecov[bot]]

Codecov Report

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

Project coverage is 67.09%. Comparing base (6cf6013) to head (203de38).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2523 +/- ## ========================================= Coverage 67.09% 67.09% Complexity 1472 1472 ========================================= Files 219 219 Lines 12628 12628 Branches 1526 1526 ========================================= Hits 8473 8473 Misses 3628 3628 Partials 527 527 ```

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