add docs example

Author: NKNaN

python/paddle/tensor/manipulation.py

@@ -1962,6 +1962,8 @@ def stack(x, axis=0, name=None):
  tensor is [N, A, B]; if ``axis == 1``, the shape of stacked
  tensor is [A, N, B], etc.
 
+ It also supports the operation with zero-size tensors which contain 0 in their shape.
+ See the examples below.
 
  .. code-block:: text
 
@@ -2005,6 +2007,38 @@ def stack(x, axis=0, name=None):
  [3.0, 4.0]
  [5.0, 6.0] ] ]
 
+
+ Case 3:
+
+ Input:
+ x[0].shape = [0, 1, 2]
+ x[0].data = []
+ x[1].shape = [0, 1, 2]
+ x[1].data = []
+
+ Attrs:
+ axis = 0
+
+ Output:
+ Out.shape = [2, 0, 1, 2]
+ Out.data = []
+
+
+ Case 4:
+
+ Input:
+ x[0].shape = [0, 1, 2]
+ x[0].data = []
+ x[1].shape = [0, 1, 2]
+ x[1].data = []
+
+ Attrs:
+ axis = 1
+
+ Output:
+ Out.shape = [0, 2, 1, 2]
+ Out.data = []
+
  Args:
  x (list[Tensor]|tuple[Tensor]): Input ``x`` can be a ``list`` or ``tuple`` of tensors, the Tensors in ``x``
  must be of the same shape and dtype. Supported data types: float32, float64, int32, int64.
@@ -2042,6 +2076,25 @@ def stack(x, axis=0, name=None):
  [[[1., 2.],
  [3., 4.],
  [5., 6.]]])
+
+ >>> # zero-size tensors
+ >>> x1 = paddle.ones([0, 1, 2])
+ >>> x2 = paddle.ones([0, 1, 2])
+
+ >>> out = paddle.stack([x1, x2], axis=0)
+ >>> print(out.shape)
+ [2, 0, 1, 2]
+ >>> print(out)
+ Tensor(shape=[2, 0, 1, 2], dtype=float32, place=Place(cpu), stop_gradient=True,
+ [[],
+ []])
+
+ >>> out = paddle.stack([x1, x2], axis=1)
+ >>> print(out.shape)
+ [0, 2, 1, 2]
+ >>> print(out)
+ Tensor(shape=[0, 2, 1, 2], dtype=float32, place=Place(cpu), stop_gradient=True,
+ [])
  """
  axis = 0 if axis is None else axis