[CustomDevice] support scalar (#45244)

paddle/phi/api/lib/scalar.cc

@@ -37,6 +37,12 @@ ScalarBase<Tensor>::ScalarBase(const Tensor& tensor_in)
     GetDataFromTensor(dst_tensor);
   } else if (tensor_in_place == phi::AllocationType::CPU) {
     GetDataFromTensor(tensor_in);
+#ifdef PADDLE_WITH_CUSTOM_DEVICE
+  } else if (tensor_in_place == phi::AllocationType::CUSTOM) {
+    Tensor dst_tensor;
+    copy(tensor_in, phi::CPUPlace(), true, &dst_tensor);
+    GetDataFromTensor(dst_tensor);
+#endif
   } else {
     PADDLE_THROW(phi::errors::Unimplemented(
         "Now, it is not supported to construct Scalar using tensor that its "

python/paddle/fluid/tests/custom_runtime/test_custom_cpu_plugin.py

@@ -41,6 +41,7 @@ class TestCustomCPUPlugin(unittest.TestCase):
             self._test_eager_backward_api()
             self._test_eager_copy_to()
             self._test_fallback_kernel()
+            self._test_scalar()
         self._test_custom_device_dataloader()
         self._test_custom_device_mnist()
 
@@ -170,6 +171,13 @@ class TestCustomCPUPlugin(unittest.TestCase):
         z = paddle.add(x, y)
         np.testing.assert_array_equal(z, r)
 
+    def _test_scalar(self):
+        import paddle
+        data_1 = paddle.to_tensor([[[[1.0, 4.0, 5.0, 7.0], [3.0, 4.0, 5.0,
+                                                            6.0]]]])
+        k_t = paddle.to_tensor([3], dtype="int32")
+        value_1, indices_1 = paddle.topk(data_1, k=k_t)
+
     def tearDown(self):
         del os.environ['CUSTOM_DEVICE_ROOT']