ReductionOperationsConverter

Converter for reduction operations.

This converter handles reduction operations using StableHLO primitives:

• sum: using stablehlo.custom_call @reduce_sum with dimension attributes

• mean: using reduce_sum + divide by element count

• variance: using stablehlo.custom_call @reduce_variance

Uses custom_call for reduction operations that require region bodies, consistent with the approach used by NeuralNetOperationsConverter for layerNorm.