[Prim] Fix get var in prim when list of single tensor (#56114)

* fix get var in prim

* fix stack test case

python/paddle/incubate/autograd/utils.py

@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import collections
 import typing
 
 import paddle
@@ -132,11 +133,13 @@ INT_DTYPE_2_STRING = {
 }
 
 
-def get_var_block(block, names):
+def get_var_block(block, names, is_tensor_list=None):
     assert isinstance(names, list)
     if len(names) == 0:
         return None
     elif len(names) == 1:
+        if is_tensor_list:
+            return [block.var(names[0])]
         return block.var(names[0])
     else:
         return [block.var(name) for name in names]
@@ -179,7 +182,7 @@ def _get_args_values(op, phi_name):
     "get attrs' values for api args' values"
     args = op_info[phi_name]
     args_list = args["args"].split(",")
-    inputs = []
+    inputs = collections.OrderedDict()
     attrs = []
 
     for item in args_list:
@@ -212,9 +215,9 @@ def _get_args_values(op, phi_name):
                 "inputs" in op_content.keys()
                 and arg_name in op_content["inputs"].keys()
             ):
-                inputs.append(op_content["inputs"][arg_name])
+                inputs[op_content["inputs"][arg_name]] = arg_type
             else:
-                inputs.append(arg_name)
+                inputs[arg_name] = arg_type
         else:
             attr_value = _get_attr_value(op, arg_type, arg_name)
             attrs.append(attr_value)
@@ -237,8 +240,15 @@ def prepare_python_api_arguments(op):
             phi_name = op.type
         inputs, attrs = _get_args_values(op, phi_name)
         res = []
-        for item in inputs:
+        for item, tensor_type in inputs.items():
             if item in op.input_names:
+                if tensor_type == "Tensor[]":
+                    res.append(
+                        get_var_block(
+                            op.block, op.input(item), is_tensor_list=True
+                        )
+                    )
+                else:
                     res.append(get_var_block(op.block, op.input(item)))
             else:
                 # Note: in some cases, inputs may be optional, thus assign None. Such case must be recorded.

test/legacy_test/test_stack_op.py

@@ -375,5 +375,20 @@ class TestStackAPI_ZeroDim(unittest.TestCase):
         paddle.enable_static()
 
 
+class TestStackListOfSingleTensor(unittest.TestCase):
+    def setUp(self):
+        paddle.disable_static()
+        paddle.seed(2022)
+        self.x = [paddle.randn((4, 2, 6), dtype="float32")]
+
+    def test_list_single_tensor(self):
+        expect = paddle.stack(self.x)
+        paddle.fluid.core._set_prim_all_enabled(True)
+        st_model = paddle.jit.to_static(paddle.stack)
+        actual = st_model(self.x)
+        np.testing.assert_allclose(expect, actual)
+        paddle.enable_static()
+
+
 if __name__ == '__main__':
     unittest.main()