Improve correctness of stddev and variance with partial aggregation

[rschlussel]

Description

When merging varianceStates for partial aggregation, if the current state has zero rows, use the values from the other state without doing computation. This prevents introducing error due to imprecision in floating point numbers.

Additionally, change the way we combine means. This ensures that we do not introduce error due to imprecision in multiplication/division when the delta is 0. I think it should in general improve the error introduced by the mean computation, but I don't have a rigorous proof or even experimental data for this.

Motivation and Context

Queries with 0 variance on large values can return inconsistent and incorrect stddev due to error introduced by floating point arithmetic. For example, see the following result for a stddev and variance computations over a constant.

presto> SELECT count(*), stddev(6523763181031200), variance(6523763181031200) from test_table;
  _col0   |       _col1        |      _col2       
----------+--------------------+------------------
 61782553 | 2.2677238191332383 | 5.14257131986424 
(1 row)

that same query with partial aggregation disabled returns correct results

presto> set session prefer_partial_aggregation=false;
SET SESSION           
presto> SELECT count(*), stddev(6523763181031200), variance(6523763181031200) from test_table;
  _col0   | _col1 | _col2 
----------+-------+-------
 61782553 |   0.0 |   0.0 
(1 row)

This change reduces the amount of error we introduce in merging variance states during partial aggregation for certain cases to improve the accuracy of our variance and stddev functions.

Impact

Ensures that when variance or stddev is zero, results are always correct, and reduces the error we introduce for other cases.

Test Plan

new unit tests
production verifier run (in progress)

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Release Notes

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== RELEASE NOTES ==

General Changes
* Improve stddev and variance functions to always return correct results when input is constant. :pr:`23447`

[steveburnett]

Nit: suggest a minor edit to release notes entry following the Order of Changes in the Release Notes Guidelines, based on the commit message. Please modify my suggestion if you think of a better wording!

== RELEASE NOTES ==

General Changes
* Improve stddev and variance functions to always return correct results when input is constant. :pr:`23447`

[amitkdutta]

Looks great. Thanks @rschlussel
Additionally, this will remove verification noise between native and java engines, as native engine computes it properly today with identical values for statistical aggregates (e.g. stddev, variance)

[zacw7]

Thanks for the prompt fix!