WebNN: Fix constant-only graph compute failure for DirectML backend

Some graphs may only have constant operands as graph input. For such
graphs, DirectML backend won’t need to allocate upload and input GPU
buffers during graph initialization, and copy the input data to those
buffers during graph computing.

This CL also adds the unit tests that build and compute constant-only
graphs for both DirectML and XNNPACK backends.

Bug: 1513582,1273291
Change-Id: I9adb7d661468e0764f7b43be2f0e062a7e989ce5
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5143695
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Commit-Queue: ningxin hu <ningxin.hu@intel.com>
Cr-Commit-Position: refs/heads/main@{#1242889}