[js/webgpu] fix bcast in where (microsoft#19009)

Author: guschmue

js/web/lib/wasm/jsep/webgpu/ops/where.ts

@@ -76,7 +76,6 @@ const createWhereOpProgramInfo = (inputs: readonly TensorView[]): ProgramInfo =>
   const isBroadcast = !(ShapeUtil.areEqual(dimsA, dimsB) && ShapeUtil.areEqual(dimsB, dimsC));
   let outputShape = dimsA;
   let outputSize = ShapeUtil.size(dimsA);
-  const vecSize = Math.ceil(outputSize / 4);
   // TODO: deal with zero-sized tensors (eg. dims=[1,0])
 
   if (isBroadcast) {
@@ -88,6 +87,8 @@ const createWhereOpProgramInfo = (inputs: readonly TensorView[]): ProgramInfo =>
     outputSize = ShapeUtil.size(outputShape);
   }
 
+  const vecSize = Math.ceil(outputSize / 4);
+
   return {
     name: 'Where',
     shaderCache: {inputDependencies: ['rank', 'rank', 'rank']},