[Fix IndexError] add unstack axis check (PaddlePaddle#49943)

* add unstack axis check

* IndexErr -> ValueError

* add static select

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

@@ -84,5 +84,29 @@ def initParameters(self):
         self.axis = 2
 
 
+class TestUnstackZeroInputOp(unittest.TestCase):
+    def unstack_zero_input_static(self):
+
+        paddle.enable_static()
+
+        array = np.array([], dtype=np.float32)
+        x = paddle.to_tensor(np.reshape(array, [0]), dtype='float32')
+        paddle.unstack(x, axis=1)
+
+    def unstack_zero_input_dynamic(self):
+
+        array = np.array([], dtype=np.float32)
+        x = paddle.to_tensor(np.reshape(array, [0]), dtype='float32')
+        paddle.unstack(x, axis=1)
+
+    def test_type_error(self):
+        paddle.disable_static()
+
+        self.assertRaises(ValueError, self.unstack_zero_input_dynamic)
+        self.assertRaises(ValueError, self.unstack_zero_input_static)
+
+        paddle.disable_static()
+
+
 if __name__ == '__main__':
     unittest.main()

python/paddle/tensor/manipulation.py

@@ -543,6 +543,10 @@ def unstack(x, axis=0, num=None):
             y = paddle.unstack(x, axis=1)  # unstack with second axis, which results 3 tensors with shape=[2, 5]
 
     """
+    if not (-x.ndim <= axis < x.ndim):
+        raise ValueError(
+            '`axis` must be in the range [-{0}, {0})'.format(x.ndim)
+        )
     if in_dygraph_mode():
         if num is None:
             num = x.shape[axis]