Fix _numel func logic and add test (#37810)

paddle/fluid/pybind/imperative.cc

@@ -1963,10 +1963,6 @@ void BindImperative(py::module *m_ptr) {
       .def("_numel",
            [](std::shared_ptr<imperative::VarBase> &self) {
              auto *t = self->MutableVar()->GetMutable<framework::LoDTensor>();
-             PADDLE_ENFORCE_EQ(
-                 t->IsInitialized(), true,
-                 platform::errors::InvalidArgument(
-                     "Tensor %s has not been initialized!", self->Name()));
              return t->numel();
            })
       .def_property("name", &imperative::VarBase::Name,

python/paddle/fluid/tests/unittests/test_var_base.py

@@ -1279,7 +1279,7 @@ class TestVarBaseInitVarBaseFromTensorWithDevice(unittest.TestCase):
 
 
 class TestVarBaseNumel(unittest.TestCase):
-    def test_numel(self):
+    def test_numel_normal(self):
         paddle.disable_static()
         np_x = np.random.random((3, 8, 8))
         x = paddle.to_tensor(np_x, dtype="float64")
@@ -1287,6 +1287,12 @@ class TestVarBaseNumel(unittest.TestCase):
         x_expected_numel = np.product((3, 8, 8))
         self.assertEqual(x_actual_numel, x_expected_numel)
 
+    def test_numel_without_holder(self):
+        paddle.disable_static()
+        x_without_holder = core.VarBase()
+        x_actual_numel = x_without_holder._numel()
+        self.assertEqual(x_actual_numel, 0)
+
 
 class TestVarBaseCopyGradientFrom(unittest.TestCase):
     def test_copy_gradient_from(self):