[GPU] allow scalar eltwise primitive fusion with gemm (#28764)

### Details:
- allows fusion of scalar eltwise layers with gemm to prevent unfusion
for some LLMs

### Tickets:
 - 161678

src/plugins/intel_gpu/src/graph/primitive_inst.cpp

@@ -2672,14 +2672,16 @@ bool primitive_inst::is_valid_fusion() const {
         const auto& outer_dep = _deps[outer_dep_idx];
 
         const auto& outer_dep_pshape = outer_dep.first->_impl_params->get_output_layout().get_partial_shape();
+        size_t outer_dep_pshape_count = outer_dep_pshape.is_static() ? ov::shape_size(outer_dep_pshape.to_shape()) : 0;
         auto merged_shape = out_pshape;
         bool can_broadcast = true;
         if (fd.is_type<eltwise>())
             can_broadcast = ov::PartialShape::broadcast_merge_into(merged_shape, outer_dep_pshape, fd.typed_desc<eltwise>()->broadcast_spec);
 
         // Check if broadcast happens more than single axis.
         // Current gemm_tiled_opt kernel FUSED_OP_LOAD macro cannot support broadcast on dynamic dimension.
-        if (_node->is_type<gemm>() && can_broadcast == true && merged_shape.rank().get_length() >= outer_dep_pshape.rank().get_length()) {
+        if (_node->is_type<gemm>() && can_broadcast == true && merged_shape.rank().get_length() >= outer_dep_pshape.rank().get_length() &&
+            outer_dep_pshape_count != 1) {
             uint8_t broadcast_more_than_single_axis = 0;
             auto updated_outer_dep_pshape = ov::PartialShape(outer_dep_pshape);
 
@@ -2715,7 +2717,7 @@ bool primitive_inst::is_valid_fusion() const {
                                                          cldnn::format::dimension(data_layout.format),
                                                          false);
 
-            if (gemm_dims[0] != data_dims[0])
+            if (gemm_dims[0] != data_dims[0] && outer_dep_pshape_count != 1)
                 return false;
         } else if (_node->is_type<fully_connected>() && _node->get_preferred_impl_type() == impl_types::onednn) {
             const auto& fc_layout = _impl_params->get_output_layout();

src/plugins/intel_gpu/tests/unit/fusions/gemm_fusion_test.cpp

@@ -432,6 +432,40 @@ TEST_P(gemm_2in_add, eltwise_postop_scalar) {
     execute(p, false, true);
 }
 
+TEST_P(gemm_2in_add, eltwise_postop_scalar_dynamic) {
+    auto p = GetParam();
+
+    if (engine.get_device_info().supports_immad) {
+        ov::intel_gpu::ImplementationDesc gemmv_impl = { cldnn::format::type::any, "", impl_types::onednn };
+        cfg_fused.set_property(ov::intel_gpu::force_implementations(ov::intel_gpu::ImplForcingMap{ { "gemm_prim", gemmv_impl } }));
+        cfg_fused.set_property(ov::intel_gpu::use_only_static_kernels_for_dynamic_shape(true));
+    }
+
+    auto add_data_layout = get_output_layout(p);
+    auto add_data_size = add_data_layout.get_partial_shape();
+    for (size_t i = 0; i < add_data_size.size(); i++)
+        add_data_size[i] = 1;
+    add_data_layout.set_partial_shape(add_data_size);
+
+    auto in_layout0 = get_input_layout(p, 0);
+    auto in_layout1 = get_input_layout(p, 1);
+
+    in_layout0.set_partial_shape(ov::PartialShape::dynamic(p.in_shapes[0].size()));
+    in_layout1.set_partial_shape(ov::PartialShape::dynamic(p.in_shapes[1].size()));
+
+    create_topologies(
+        input_layout("input0", in_layout0),
+        input_layout("input1", in_layout1),
+        data("add_data", get_mem(add_data_layout, 0.5f)),
+        gemm("gemm_prim", { input_info("input0"), input_info("input1") }, data_types::f32, false, false, 1.f, 0.f, in_layout0.get_rank(), in_layout1.get_rank()),
+        eltwise("add_prim", { input_info("gemm_prim"), input_info("add_data") }, p.eltwise_m, p.default_type),
+        reorder("reorder_bfyx", input_info("add_prim"), p.default_format, data_types::f32)
+    );
+
+    tolerance = default_tolerance(p.default_type);
+    execute(p, true, true);
+}
+
 INSTANTIATE_TEST_SUITE_P(fusings_gpu, gemm_2in_add, ::testing::ValuesIn(std::vector<gemm_test_params>{
     // gemm_test_params{ CASE_GEMM_2IN_FP16_3, 3, 4, "", broadcast_kinds::none, eltwise_mode::sum },    // TODO: check why failed in eltwise_postop_dynamic
     gemm_test_params{ CASE_GEMM_2IN_FP16_4, 3, 4, "", broadcast_kinds::none, eltwise_mode::sum },