refine eng doc for hard_sigmoid op (#20442)

* refine eng doc for hard_sigmoid op
test=develop
test=document_fix

* refine the description of hard_sigmoid
 test=develop
test=document_fix

* update API.spec
test=document_fix

* Refine the decription of parameters of HardSigmoid op
test=develop, test=document_fix

* Update API.spec for hard_sigmoid op
test=develop, test=document_fix

paddle/fluid/API.spec

@@ -227,7 +227,7 @@ paddle.fluid.layers.elu (ArgSpec(args=['x', 'alpha', 'name'], varargs=None, keyw
 paddle.fluid.layers.relu6 (ArgSpec(args=['x', 'threshold', 'name'], varargs=None, keywords=None, defaults=(6.0, None)), ('document', '56d3bb2e38cd4de769267c0ace5bbac2'))
 paddle.fluid.layers.pow (ArgSpec(args=['x', 'factor', 'name'], varargs=None, keywords=None, defaults=(1.0, None)), ('document', '00d437d1e0d9450ea75a0495b93b54a7'))
 paddle.fluid.layers.stanh (ArgSpec(args=['x', 'scale_a', 'scale_b', 'name'], varargs=None, keywords=None, defaults=(0.67, 1.7159, None)), ('document', 'd3f742178a7263adf5929153d104883d'))
-paddle.fluid.layers.hard_sigmoid (ArgSpec(args=['x', 'slope', 'offset', 'name'], varargs=None, keywords=None, defaults=(0.2, 0.5, None)), ('document', '607d79ca873bee40eed1c79a96611591'))
+paddle.fluid.layers.hard_sigmoid (ArgSpec(args=['x', 'slope', 'offset', 'name'], varargs=None, keywords=None, defaults=(0.2, 0.5, None)), ('document', '361e179a0b446ffce90176fae60545d6'))
 paddle.fluid.layers.swish (ArgSpec(args=['x', 'beta', 'name'], varargs=None, keywords=None, defaults=(1.0, None)), ('document', '60b4dbe35f2b47f7290e79907a4eacec'))
 paddle.fluid.layers.prelu (ArgSpec(args=['x', 'mode', 'param_attr', 'name'], varargs=None, keywords=None, defaults=(None, None)), ('document', 'cb417a61f701c937f33d057fe85203ab'))
 paddle.fluid.layers.brelu (ArgSpec(args=['x', 't_min', 't_max', 'name'], varargs=None, keywords=None, defaults=(0.0, 24.0, None)), ('document', '35db66985af04bf6a91601aa4f73b54f'))

paddle/fluid/operators/activation_op.cc

@@ -572,23 +572,23 @@ class ThresholdedReluOpMaker : public framework::OpProtoAndCheckerMaker {
 class HardSigmoidOpMaker : public framework::OpProtoAndCheckerMaker {
  public:
   void Make() override {
-    AddInput("X", "Input of HardSigmoid operator");
-    AddOutput("Out", "Output of HardSigmoid operator");
-    AddAttr<float>("slope", "Slope for linear approximation of sigmoid")
+    AddInput("X", "An N-D Tensor with data type float32, float64. ");
+    AddOutput("Out", "A Tensor with the same shape as input. ");
+    AddAttr<float>("slope",
+                   "The slope of the linear approximation of sigmoid. Its "
+                   "value MUST BE positive. Default is 0.2. ")
         .SetDefault(0.2f);
-    AddAttr<float>("offset", "Offset for linear approximation of sigmoid")
+    AddAttr<float>(
+        "offset",
+        "The offset of the linear approximation of sigmoid. Default is 0.5. ")
         .SetDefault(0.5f);
     AddComment(R"DOC(
 HardSigmoid Activation Operator.
 
-Segment-wise linear approximation of sigmoid(https://arxiv.org/abs/1603.00391),
+A 3-part piecewise linear approximation of sigmoid(https://arxiv.org/abs/1603.00391),
 which is much faster than sigmoid.
 
-$out = \max(0, \min(1, slope * x + shift))$
-
-The slope should be positive. The offset can be either positive or negative.
-The default slope and shift are set according to the above reference.
-It is recommended to use the defaults for this activation.
+$out = \max(0, \min(1, slope * x + offset))$
 
 )DOC");
   }

python/paddle/fluid/layers/nn.py

@@ -11751,23 +11751,24 @@ def stanh(x, scale_a=0.67, scale_b=1.7159, name=None):
 def hard_sigmoid(x, slope=0.2, offset=0.5, name=None):
     """
     ${comment}
-    Args:
-        x(${x_type}): ${x_comment}
-        slope(${slope_type}|0.2): ${slope_comment}
-        offset(${offset_type}|0.5): ${offset_comment}
-        name(str|None): A name for this layer(optional). If set None, the layer
-                        will be named automatically.
+    Parameters:
+        x (${x_type}): ${x_comment}
+        slope (float, optional): ${slope_comment}
+        offset (float, optional): ${offset_comment}
+        name (str, optional): The default value is None. Normally there is no
+            need for user to set this property. For more information, please
+            refer to :ref:`api_guide_Name`
 
     Returns:
-        output(${out_type}): ${out_comment}
+        ${out_type}: ${out_comment}
 
     Examples:
 
         .. code-block:: python
 
             import paddle.fluid as fluid
-            x = fluid.layers.data(name="x", shape=[3,10,32,32], dtype="float32")
-            y = fluid.layers.hard_sigmoid(x, slope=0.3, offset=0.8)
+            data = fluid.layers.fill_constant(shape=[3, 2], value=0.5, dtype='float32') # [[0.5, 0.5], [0.5, 0.5], [0.5, 0.5]]
+            result = fluid.layers.hard_sigmoid(data) # [[0.6, 0.6], [0.6, 0.6], [0.6, 0.6]]
     """
     helper = LayerHelper('hard_sigmoid', **locals())
     out = helper.create_variable_for_type_inference(dtype=x.dtype)