add WeightedRandomSampler. test=develop (#2872)

Author: heavengate

doc/paddle/api/paddle/io/WeightedRandomSampler_cn.rst

@@ -0,0 +1,31 @@
+.. _cn_api_io_cn_WeightedRandomSampler:
+
+WeightedRandomSampler
+-------------------------------
+
+.. py:class:: paddle.io.WeightedRandomSampler(weights, num_samples, replacement=True)
+
+通过制定的权重随机采样，采样下标范围在 ``[0, len(weights) - 1]`` , 如果 ``replacement`` 为 ``True`` ，则下标可被采样多次
+
+参数:
+    - **weights** (numpy.ndarray|paddle.Tensor|tuple|list) - 权重序列，需要是numpy数组，paddle.Tensor，list或者tuple类型。
+    - **num_samples** (int) - 采样样本数。
+    - **replacement** (bool) - 是否采用有放回的采样，默认值为True
+
+返回: 返回根据权重随机采样下标的采样器
+
+返回类型: WeightedRandomSampler 
+
+**代码示例**
+
+.. code-block:: python
+
+    from paddle.io import WeightedRandomSampler
+
+    sampler = WeightedRandomSampler(weights=[0.1, 0.3, 0.5, 0.7, 0.2],
+                                    num_samples=5,
+                                    replacement=True)
+
+    for index in sampler:
+        print(index)
+