feat(vortex-array): union_child_validities expr helper (#8829)

## What

Adds `vortex_array::expr::union_child_validities(expr)`: the conjunction
of an expression's child validities (result is null iff any operand is
null). This is the common `ScalarFnVTable::validity()` for
null-propagating kernels (comparisons, arithmetic, most geo/tensor ops),
letting the planner derive the output null mask without executing the
kernel.

Migrates `vortex-tensor`'s four scalar functions (`InnerProduct`,
`CosineSimilarity`, `L2Denorm`, `L2Norm`) onto it, replacing their
inline `and(child validities)`.

Signed-off-by: Nemo Yu <zyu379@wisc.edu>

Author: HarukiMoriarty

vortex-array/src/expr/exprs.rs

@@ -7,6 +7,7 @@ use std::sync::Arc;
 use std::sync::LazyLock;
 
 use vortex_error::VortexExpect;
+use vortex_error::VortexResult;
 use vortex_error::vortex_panic;
 use vortex_utils::iter::ReduceBalancedIterExt;
 
@@ -434,6 +435,22 @@ where
     iter.into_iter().reduce_balanced(and)
 }
 
+/// The conjunction of an expression's child validities — i.e. the validity of a scalar function
+/// whose result is null exactly when any operand is null.
+///
+/// This is the `ScalarFnVTable::validity` for kernels that propagate nulls and never produce a
+/// null from non-null inputs (comparisons, arithmetic, most geo and tensor ops). Returning it lets
+/// the planner derive the output's null mask without executing the kernel. Yields `None` when the
+/// expression has no children.
+pub fn union_child_validities(expression: &Expression) -> VortexResult<Option<Expression>> {
+    let child_validities = expression
+        .children()
+        .iter()
+        .map(Expression::validity)
+        .collect::<VortexResult<Vec<_>>>()?;
+    Ok(and_collect(child_validities))
+}
+
 /// Create a new [`Binary`] using the [`Add`](Operator::Add) operator.
 ///
 /// ## Example usage

vortex-tensor/src/scalar_fns/cosine_similarity.rs

@@ -15,7 +15,7 @@ use vortex_array::arrays::scalar_fn::plugin::ScalarFnArrayVTable;
 use vortex_array::dtype::DType;
 use vortex_array::dtype::Nullability;
 use vortex_array::expr::Expression;
-use vortex_array::expr::and;
+use vortex_array::expr::union_child_validities;
 use vortex_array::match_each_float_ptype;
 use vortex_array::scalar_fn::Arity;
 use vortex_array::scalar_fn::ChildName;
@@ -180,10 +180,7 @@ impl ScalarFnVTable for CosineSimilarity {
         expression: &Expression,
     ) -> VortexResult<Option<Expression>> {
         // The result is null if either input tensor is null.
-        let lhs_validity = expression.child(0).validity()?;
-        let rhs_validity = expression.child(1).validity()?;
-
-        Ok(Some(and(lhs_validity, rhs_validity)))
+        union_child_validities(expression)
     }
 
     fn is_null_sensitive(&self, _options: &Self::Options) -> bool {

vortex-tensor/src/scalar_fns/inner_product.rs

@@ -18,7 +18,7 @@ use vortex_array::dtype::DType;
 use vortex_array::dtype::NativePType;
 use vortex_array::dtype::Nullability;
 use vortex_array::expr::Expression;
-use vortex_array::expr::and;
+use vortex_array::expr::union_child_validities;
 use vortex_array::match_each_float_ptype;
 use vortex_array::scalar_fn::Arity;
 use vortex_array::scalar_fn::ChildName;
@@ -164,10 +164,7 @@ impl ScalarFnVTable for InnerProduct {
         expression: &Expression,
     ) -> VortexResult<Option<Expression>> {
         // The result is null if either input tensor is null.
-        let lhs_validity = expression.child(0).validity()?;
-        let rhs_validity = expression.child(1).validity()?;
-
-        Ok(Some(and(lhs_validity, rhs_validity)))
+        union_child_validities(expression)
     }
 
     fn is_null_sensitive(&self, _options: &Self::Options) -> bool {

vortex-tensor/src/scalar_fns/l2_denorm.rs

@@ -31,7 +31,7 @@ use vortex_array::dtype::NativePType;
 use vortex_array::dtype::Nullability;
 use vortex_array::dtype::proto::dtype as pb;
 use vortex_array::expr::Expression;
-use vortex_array::expr::and;
+use vortex_array::expr::union_child_validities;
 use vortex_array::match_each_float_ptype;
 use vortex_array::scalar::Scalar;
 use vortex_array::scalar::ScalarValue;
@@ -256,10 +256,7 @@ impl ScalarFnVTable for L2Denorm {
         _options: &Self::Options,
         expression: &Expression,
     ) -> VortexResult<Option<Expression>> {
-        let normalized_validity = expression.child(0).validity()?;
-        let norms_validity = expression.child(1).validity()?;
-
-        Ok(Some(and(normalized_validity, norms_validity)))
+        union_child_validities(expression)
     }
 
     fn is_null_sensitive(&self, _options: &Self::Options) -> bool {

vortex-tensor/src/scalar_fns/l2_norm.rs

@@ -25,6 +25,7 @@ use vortex_array::dtype::NativePType;
 use vortex_array::dtype::Nullability;
 use vortex_array::dtype::proto::dtype as pb;
 use vortex_array::expr::Expression;
+use vortex_array::expr::union_child_validities;
 use vortex_array::match_each_float_ptype;
 use vortex_array::scalar::Scalar;
 use vortex_array::scalar_fn::Arity;
@@ -184,7 +185,7 @@ impl ScalarFnVTable for L2Norm {
         expression: &Expression,
     ) -> VortexResult<Option<Expression>> {
         // The result is null if the input tensor is null.
-        Ok(Some(expression.child(0).validity()?))
+        union_child_validities(expression)
     }
 
     fn is_null_sensitive(&self, _options: &Self::Options) -> bool {