webgpu: Fix wgsl failure from MatMulSplitKProgram (#6838)

Fix that Tint WGSL reader failure: index 3 out of bounds [0..2] from MatMulSplitKProgram.

tfjs-backend-webgpu/src/webgpu_program.ts

@@ -629,6 +629,13 @@ function getOutputCoordsSnippet(
   const {x, y = [], z = []} = dispatchLayout;
 
   const outRank = outShape.length;
+  const rank = x.length + y.length + z.length;
+  // getOutputCoords is only meaningful when the output rank is same with
+  // dispatch layout rank.
+  if (rank !== outRank) {
+    return '';
+  }
+
   if (x.length === outRank) {
     const dtype = getCoordsDataType(outRank);
     const snippet = `fn getOutputCoords() -> ${dtype}{
@@ -642,17 +649,13 @@ function getOutputCoordsSnippet(
   let gatherDimensionsStr = '';
   const dims = [x, y, z];
 
-  let rank = 0;
-
   for (let i = 0; i < dims.length; i++) {
     const arr = dims[i];
 
     if (arr.length === 0) {
       continue;
     }
 
-    rank += arr.length;
-
     if (arr.length === 1) {
       gatherDimensionsStr += `let d${arr[0]} = i32(globalId[${i}]);`;
     } else {