support flip 0D (#49460)

paddle/phi/kernels/gpu/flip_kernel.cu

@@ -101,6 +101,9 @@ void FlipKernel(const Context& dev_ctx,
                 DenseTensor* out) {
   const size_t total_dims = x.dims().size();
   switch (total_dims) {
+    case 0:
+      LaunchFlipCudaKernel<T, Context, 0>(dev_ctx, x, axis, out);
+      break;
     case 1:
       LaunchFlipCudaKernel<T, Context, 1>(dev_ctx, x, axis, out);
       break;

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

@@ -454,6 +454,15 @@ class TestSundryAPI(unittest.TestCase):
         paddle.disable_static()
         self.x = paddle.rand([])
 
+    def test_flip(self):
+        x = paddle.rand([])
+        x.stop_gradient = False
+        out = paddle.flip(x, axis=[])
+        out.backward()
+        self.assertEqual(x.shape, [])
+        self.assertEqual(out.shape, [])
+        self.assertEqual(out.grad.shape, [])
+
     def test_linear(self):
         x = paddle.randn([3, 2])
         w = paddle.full(shape=[2, 4], fill_value=0.5)
@@ -753,6 +762,18 @@ class TestSundryAPIStatic(unittest.TestCase):
         paddle.enable_static()
         self.exe = paddle.static.Executor()
 
+    @prog_scope()
+    def test_flip(self):
+        x = paddle.rand([])
+        x.stop_gradient = False
+        out = paddle.flip(x, axis=[])
+        paddle.static.append_backward(out)
+
+        program = paddle.static.default_main_program()
+        res1, res2 = self.exe.run(program, fetch_list=[x, out])
+        self.assertEqual(res1.shape, ())
+        self.assertEqual(res2.shape, ())
+
     @prog_scope()
     def test_pow_factor(self):
         x = paddle.rand([])