WebNN: Flatten adjacent dimensions for GEMM > 4D for DirectML
This CL flattens adjacent dimensions for GEMM > 4D because
DML_GEMM_OPERATOR_DESC[1] restricts tensor's rank <= 4.
[1] https://learn.microsoft.com/en-us/windows/win32/api/directml/ns-directml-dml_gemm_operator_desc
Bug: 353856233
Change-Id: Ifbced3d3081594ec6fdefc3a2d381ba675aba7cf
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5810975
Reviewed-by: ningxin hu ningxin.hu@intel.com
Commit-Queue: Mingming1 Xu mingming1.xu@intel.com
Reviewed-by: Rafael Cintron rafael.cintron@microsoft.com
Cr-Commit-Position: refs/heads/main@{#1354311}