webnn: Enforce input data type constraints for gather indices

As specified in webmachinelearning/webnn#646

Bug: 328567884
Change-Id: I33eba7e1def430b1cb94e3e7a4868e82c5bbd9a3

Author: lisa0314

webnn/validation_tests/gather.https.any.js

@@ -24,15 +24,23 @@ const tests = [
     indices: {dataType: 'int64', dimensions: [1]}
   },
   {
-    name: '[gather] TypeError is expected if the axis is greater than the rank of input',
+    name:
+        '[gather] TypeError is expected if the axis is greater than the rank of input',
     input: {dataType: 'float16', dimensions: [1, 2, 3]},
     indices: {dataType: 'int32', dimensions: [5, 6]},
     axis: 4
   },
   {
-    name: '[gather] TypeError is expected if the data type of indices is invalid',
+    name:
+        '[gather] TypeError is expected if the data type of indices is float32 which is invalid',
     input: {dataType: 'float16', dimensions: [1, 2, 3, 4]},
     indices: {dataType: 'float32', dimensions: [5, 6]}
+  },
+  {
+    name:
+        '[gather] TypeError is expected if the data type of indices is int32 which is invalid',
+    input: {dataType: 'float16', dimensions: [1, 2, 3, 4]},
+    indices: {dataType: 'int32', dimensions: [5, 6]}
   }
 ];