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
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5639505
Reviewed-by: ningxin hu ningxin.hu@intel.com
Reviewed-by: Phillis Tang phillis@chromium.org
Reviewed-by: Koji Ishii kojii@chromium.org
Commit-Queue: Austin Sullivan asully@chromium.org
Cr-Commit-Position: refs/heads/main@{#1344800}