add new empty op (PR #26659) doc (#2571)

Co-authored-by: Nroot <root@yq01-qianmo-com-255-129-13.yq01.baidu.com>

doc/paddle/api/paddle/tensor/creation/empty_cn.rst

+.. _cn_api_tensor_empty:
+
+empty
+-------------------------------
+
+.. py:function:: paddle.empty(shape, dtype=None, name=None)
+
+
+
+该OP创建形状大小为shape并且数据类型为dtype的Tensor，其中元素值是未初始化的。
+
+参数：
+    - **shape** (list|tuple|Tensor) – 指定创建Tensor的形状(shape), 数据类型为int32 或者int64。
+    - **dtype** （np.dtype|str， 可选）- 输出变量的数据类型，可以是bool, float16, float32, float64, int32, int64。若为None，则输出变量的数据类型为系统全局默认类型，默认值为None。
+    - **name** （str，可选）- 具体用法请参见 :ref:`api_guide_Name`，一般无需设置，默认值为None。
+    
+返回：返回一个根据``shape``和``dtype``创建并且尚未初始化的Tensor。
+
+**代码示例**：
+
+.. code-block:: python
+
+    import paddle
+    import numpy as np
+
+    paddle.disable_static()   # Now we are in imperative mode
+    paddle.set_device("cpu")  # and use cpu device
+
+    # example 1: argument ``shape`` is a list which doesn't contain Tensor.
+    data1 = paddle.empty(shape=[2,3], dtype='float32')
+    #[[4.3612203e+27 1.8176809e+31 1.3555911e-19]     # uninitialized
+    # [1.1699684e-19 1.3563156e-19 3.6408321e-11]]    # uninitialized
+
+    # example 2: argument ``shape`` is a Tensor, the data type must be int64 or int32.
+    shape_data = np.array([2, 3]).astype('int32')
+    shape = paddle.to_tensor(shape_data)
+    data2 = paddle.empty(shape=shape, dtype='float32')
+    #[[1.7192326e-37 4.8125365e-38 1.9866003e-36]     # uninitialized
+    # [1.3284029e-40 7.1117408e-37 2.5353012e+30]]    # uninitialized
+
+    # example 3: argument ``shape`` is a list which contains Tensor.
+    dim2_data = np.array([3]).astype('int32')
+    dim2 = paddle.to_tensor(dim2_data)
+    data3 = paddle.empty(shape=[2, dim2], dtype='float32')
+    #[[1.1024214e+24 7.0379409e+22 6.5737699e-34]     # uninitialized
+    # [7.5563101e+31 7.7130405e+31 2.8020654e+20]]    # uninitialized