fix xpu unittest issue (#49760)

* fix xpu unittest issue: zero_dim_tensor

* deal with leftout issue introduced by #49470

paddle/phi/backends/xpu/xpu2_op_list.cc

@@ -269,12 +269,6 @@ XPUOpMap& get_kl2_ops() {
                      phi::DataType::INT8,
                      phi::DataType::FLOAT16,
                      phi::DataType::FLOAT32})},
-      {"flatten",
-       XPUKernelSet({phi::DataType::INT64,
-                     phi::DataType::INT32,
-                     phi::DataType::INT8,
-                     phi::DataType::FLOAT16,
-                     phi::DataType::FLOAT32})},
       {"flatten_grad",
        XPUKernelSet({phi::DataType::INT64,
                      phi::DataType::INT32,

python/paddle/fluid/tests/unittests/xpu/test_zero_dim_tensor_xpu.py

@@ -747,6 +747,7 @@ class TestSundryAPI(unittest.TestCase):
             out1.numpy(),
         )
 
+        out0.retain_grads()
         out0.backward()
         self.assertEqual(out0.grad.shape, [1])
         self.assertEqual(logit.grad.shape, [2, 3])