Clarify Kahan condition (#28790)
This pull request refactors the logic for determining when to use Kahan
compensated summation in reduction aggregators for integer types in
`onnxruntime/core/providers/cpu/reduction/reduction_ops.h`. Instead of
relying on the size of the type, a new type trait, `kReduceUseKahan<T>`,
is introduced to explicitly indicate when Kahan summation should be used
(currently for `int64_t` and `uint64_t`). This improves code clarity and
maintainability by making the condition more explicit and less
error-prone.
**Refactoring of Kahan Summation Condition:**
* Introduced the `kReduceUseKahan<T>` constexpr variable to indicate
when Kahan compensated summation should be used, replacing the previous
`sizeof(T) >= sizeof(int64_t)` checks. This trait currently evaluates to
true for `int64_t` and `uint64_t` types.
* Updated all reduction aggregator classes (`ReduceAggregatorSum`,
`ReduceAggregatorSumSquare`, `ReduceAggregatorMean`,
`ReduceAggregatorL1`, `ReduceAggregatorL2`, `ReduceAggregatorLogSum`) to
use `kReduceUseKahan<T>` instead of the previous size-based condition
when deciding to apply Kahan summation in both `aggall` and `update`
methods.
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