[WebNN EP] Fix bug introduced by output rank validation (#32067)
The output rank validation added in #31708 wrongly falls back ops whose
ONNX output rank differs from the WebNN operand rank:
- InstanceNormalization, Conv (conv1d) and MatMul (1-D input) reshape
the input to a WebNN-valid rank and reshape the output back. Skip the
output rank check exactly when that workaround fires, mirroring the
existing input-side skips. Also make the InstanceNormalization
input-side check symmetric (only validate the 4D direct path).
- Gather with 0-D indices yields a valid 0-D output, but the WebNN
gather output rankRange is reported with min=1; permit 0-D output.