[WebNN EP] Fix bug in Softmax (microsoft#17665)

### Description
<!-- Describe your changes. -->
For now, WebNN Softmax only support 2D (or implicitly coerce to 2D)
inputs and the last axis.


### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Fallback some cases to pass the CI.

onnxruntime/core/providers/webnn/builders/impl/softmax_op_builder.cc

@@ -37,13 +37,18 @@ Status SoftmaxOpBuilder::AddToModelBuilderImpl(ModelBuilder& model_builder,
   const auto input_size = input_shape.size();
   // WebNN Softmax only support 2d input shape, reshape input to 2d.
   if (input_size != 2) {
-    int32_t new_shape_0 = SafeInt<int32_t>(input_shape.data()[0]);
-    for (size_t i = 1; i < input_size - 1; i++) {
-      new_shape_0 *= input_shape.data()[i];
-    }
-    emscripten::val new_shape = emscripten::val::array();
-    new_shape.call<void>("push", new_shape_0);
-    new_shape.call<void>("push", static_cast<int32_t>(input_shape.back()));
+    NodeAttrHelper helper(node);
+    int32_t axis = helper.Get("axis", 1);
+    if (node.SinceVersion() >= 13)
+      // Opset 13 has default value -1.
+      axis = helper.Get("axis", -1);
+    //  Coerce the input into a 2-dimensional tensor with dimensions [a_0 * ... * a_{k-1}, a_k * ... * a_{n-1}].
+    axis = static_cast<int32_t>(HandleNegativeAxis(axis, input_size));
+    int32_t first_dim = static_cast<int32_t>(std::reduce(input_shape.begin(), input_shape.begin() + axis,
+                                                         1, std::multiplies<int64_t>()));
+    int32_t second_dim = static_cast<int32_t>(std::reduce(input_shape.begin() + axis, input_shape.end(),
+                                                          1, std::multiplies<int64_t>()));
+    emscripten::val new_shape = emscripten::val::array(std::vector<int32_t>{first_dim, second_dim});
     input = model_builder.GetBuilder().call<emscripten::val>("reshape", input, new_shape);
   }
   output = model_builder.GetBuilder().call<emscripten::val>("softmax", input);
@@ -76,9 +81,10 @@ bool SoftmaxOpBuilder::IsOpSupportedImpl(const InitializedTensorSet& /* initiali
     return false;
   }
   NodeAttrHelper helper(node);
-  const int32_t axis = helper.Get("axis", 1);
-  // WebNN softmax only support input axis 1
-  if (axis != 1 && axis != -1) {
+  const int64_t axis = helper.Get("axis", 1);
+  // WebNN softmax only support reshape for the last axis or version before 13.
+  // TODO: support opset 13 by composing into: Exp(input) / ReduceSum(Exp(input), axis=axis, keepdims=1).
+  if (axis != -1 && axis != input_shape.size() - 1 && node.SinceVersion() >= 13) {
     LOGS(logger, VERBOSE) << "SoftMax only support axis 1 or -1, input axis: " << axis;
     return false;
   }