Add scalar case if "then" and "else" subgraph output are scalar. (#28444

)

### Details:
- *if "then" and "else" subgraph output shape are scalar, the if node
output shape should be static and a scalar*
 - *...*

### Tickets:
 - *[28235](#28235

src/core/src/op/if.cpp

@@ -33,15 +33,19 @@ static ov::PartialShape resolve_shape(const ov::PartialShape& then_pshape, const
         return ov::PartialShape::dynamic();
     }
     if (then_rank.get_length() != else_rank.get_length()) {
+        auto is_one_element = [](const ov::PartialShape& pshape) {
+            return pshape.size() == 0 || (pshape.is_static() && pshape[0].get_length() == 1);
+        };
         // Union of scalar and 1D case
         if (then_rank.get_length() <= 1 && else_rank.get_length() <= 1) {
-            return ov::PartialShape::dynamic(1);
+            return (is_one_element(then_pshape) && is_one_element(else_pshape)) ? ov::PartialShape{1}
+                                                                                : ov::PartialShape::dynamic(1);
         } else {
             return ov::PartialShape::dynamic();
         }
     }
-    ov::PartialShape new_dims;
 
+    ov::PartialShape new_dims;
     // If ranges are equal each dimension of then_body output is union with each dimension of
     // else_body
     for (auto then_it = then_pshape.cbegin(), else_it = else_pshape.cbegin(); then_it != then_pshape.cend();

src/core/tests/type_prop/if.cpp

@@ -340,6 +340,36 @@ TEST(type_prop, if_scalar_and_1d_static_union) {
     EXPECT_EQ(sh, out_shape);
 }
 
+TEST(type_prop, if_output_one_element) {
+    // That which we iterate over
+    auto X = make_shared<ov::op::v0::Parameter>(element::f32, Shape{});
+    auto Y = make_shared<ov::op::v0::Parameter>(element::f32, Shape{1});
+    auto cond = make_shared<ov::op::v0::Parameter>(element::boolean, Shape{});
+
+    // Body parameters
+    auto Xt = make_shared<ov::op::v0::Parameter>(element::f32, PartialShape::dynamic());
+    auto Ye = make_shared<ov::op::v0::Parameter>(element::f32, PartialShape::dynamic());
+    // Body
+    auto then_op = std::make_shared<op::v1::Add>(Xt, Xt);
+    auto then_body_res = make_shared<ov::op::v0::Result>(then_op);
+    auto then_body = make_shared<ov::Model>(OutputVector{then_body_res}, ParameterVector{Xt});
+
+    auto else_op = std::make_shared<op::v1::Maximum>(Ye, Ye);
+    auto else_body_res = make_shared<ov::op::v0::Result>(else_op);
+    auto else_body = make_shared<ov::Model>(OutputVector{else_body_res}, ParameterVector{Ye});
+
+    auto if_op = make_shared<op::v8::If>(cond);
+    if_op->set_then_body(then_body);
+    if_op->set_else_body(else_body);
+    if_op->set_input(X, Xt, nullptr);
+    if_op->set_input(Y, nullptr, Ye);
+    auto res = if_op->set_output(then_body_res, else_body_res);
+    auto result0 = make_shared<ov::op::v0::Result>(res);
+    PartialShape out_shape{1};
+    auto sh = result0->get_output_partial_shape(0);
+    EXPECT_EQ(sh, out_shape);
+}
+
 TEST(type_prop, if_element_type_dynamic) {
     // That which we iterate over
     auto X = make_shared<ov::op::v0::Parameter>(element::f16, Shape{32, 40, 10});