add doc for erf op (#1675)

* add doc for erf op in Chinese.
* update doc for erf op to add float16 support.

doc/fluid/api_cn/layers_cn/erf_cn.rst

+.. _cn_api_fluid_layers_erf:
+
+erf
+-------------------------------
+
+.. py:function:: paddle.fluid.layers.erf(x)
+
+逐元素计算 Erf 激活函数。更多细节请参考 `Error function <https://en.wikipedia.org/wiki/Error_function>`_ 。
+
+
+.. math::
+    out = \frac{2}{\sqrt{\pi}} \int_{0}^{x}e^{- \eta^{2}}d\eta
+
+参数：
+  - **x** (Variable) - Erf Op 的输入，多维 Tensor 或 LoDTensor，数据类型为 float16, float32 或 float64。
+
+返回：
+  - 多维 Tensor 或 LoDTensor, 数据类型为 float16, float32 或 float64， 和输入 x 的数据类型相同，形状和输入 x 相同。
+
+返回类型：
+  - Variable
+
+**代码示例**：
+
+.. code-block:: python
+    
+    # declarative mode
+    import numpy as np
+    from paddle import fluid
+    
+    x = fluid.data(name="x", shape=(-1, 3), dtype="float32")
+    y = fluid.layers.erf(x)
+    
+    place = fluid.CPUPlace()
+    exe = fluid.Executor(place)
+    start = fluid.default_startup_program()
+    main = fluid.default_main_program()
+    
+    data = np.random.randn(2, 3).astype("float32")
+    exe.run(start)
+    
+    y_np, = exe.run(main, feed={"x": data}, fetch_list=[y])
+    
+    data
+    # array([[ 0.4643714 , -1.1509596 ,  1.2538221 ],
+    #        [ 0.34369683,  0.27478245,  1.1805398 ]], dtype=float32)
+    y_np
+    # array([[ 0.48863927, -0.8964121 ,  0.9237998 ],
+    #        [ 0.37307587,  0.30242872,  0.9049887 ]], dtype=float32)
+
+.. code-block:: python
+
+    # imperative mode
+    import numpy as np
+    from paddle import fluid
+    import paddle.fluid.dygraph as dg
+    
+    data = np.random.randn(2, 3).astype("float32")
+    place = fluid.CPUPlace()
+    with dg.guard(place) as g:
+        x = dg.to_variable(data)
+        y = fluid.layers.erf(x)
+        y_np = y.numpy()
+    data
+    # array([[ 0.4643714 , -1.1509596 ,  1.2538221 ],
+    #        [ 0.34369683,  0.27478245,  1.1805398 ]], dtype=float32)
+    y_np
+    # array([[ 0.48863927, -0.8964121 ,  0.9237998 ],
+    #        [ 0.37307587,  0.30242872,  0.9049887 ]], dtype=float32)