[WebNN] Correct behavior of Softmax < opset 13 (#26391)
ONNX Softmax operates with different semantics before and after opset
13. Before opset 13, it normalizes over the flattened range of
dimensions starting from axis to the last dimension. Fix it by reshaping
the input to [M, N] where M = prod(d0..d{axis-1}) and N =
prod(d{axis-1}..d{n-1}) and applying softmax along N, then reshaping
back to the original shape.
