webnn: Set a max operand rank of 8

We likely eventually want this to be part of MLOpSupportsLimits, but
for now this allows us to replace some checked-casts in favor of
static-casts, and not need to worry about tensors with absurd ranks

See webmachinelearning/webnn#456

Bug: 329482489
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel,mac14-blink-rel,mac14.arm64-blink-rel
Change-Id: I021d3b30ea1b8f5f3bef1725130fd2e4c569f494

webnn/validation_tests/invalid-rank.https.any.js

@@ -0,0 +1,16 @@
+// META: title=ensure an MLOperand cannot be created with an invalid rank
+// META: global=window,dedicatedworker
+// META: variant=?cpu
+// META: variant=?gpu
+// META: variant=?npu
+// META: script=../resources/utils_validation.js
+
+promise_test(async t => {
+  const builder = new MLGraphBuilder(context);
+
+  const shapeWithLargeRank = Array(10).fill(2);
+  assert_throws_js(
+      TypeError,
+      () => builder.input(
+          'a', {dataType: 'float32', dimensions: shapeWithLargeRank}));
+}, 'Throw if rank is too large');