[Accuracy diff No.79] Fix accuracy diff for paddle.combinations API (#73293)

* fix accuracy for combinations

* empty commit

Author: ooooo-create

python/paddle/tensor/math.py

@@ -8391,11 +8391,6 @@ def combinations(
     if r == 0:
         return paddle.empty(shape=[0], dtype=x.dtype)
 
-    if (r > x.shape[0] and not with_replacement) or (
-        x.shape[0] == 0 and with_replacement
-    ):
-        return paddle.empty(shape=[0, r], dtype=x.dtype)
-
     if r > 1:
         t_l = [x for i in range(r)]
         grids = paddle.meshgrid(t_l)

test/legacy_test/test_combinations.py

@@ -143,9 +143,22 @@ def test_dygraph(self):
         for place in self.place:
             paddle.device.set_device(place)
             a = paddle.rand([3], dtype='float32')
+            a.stop_gradient = False
             c = paddle.combinations(a, r=4)
             expected = convert_combinations_to_array(a.numpy(), r=4)
             np.testing.assert_allclose(c, expected)
+            loss = c.sum().backward()
+            expected = np.zeros([3], dtype='float32')
+            np.testing.assert_allclose(a.grad, expected)
+
+            a = paddle.rand([0], dtype='float32')
+            a.stop_gradient = False
+            c = paddle.combinations(a, r=2, with_replacement=True)
+            expected = convert_combinations_to_array(a.numpy(), r=2)
+            np.testing.assert_allclose(c, expected)
+            loss = c.sum().backward()
+            expected = np.empty([0], dtype='float32')
+            np.testing.assert_allclose(a.grad, expected)
 
             # test empty input
             a = paddle.empty([random.randint(0, 8)])