add softsign activation.

paddle/gserver/activations/ActivationFunction.cpp

@@ -212,6 +212,37 @@ Error __must_check backward(Argument& act) {
 }
 END_DEFINE_ACTIVATION(sequence_softmax)
 
+/*
+ * @brief SoftSign Activation.
+ * \f[
+ * f(z) = \frac{z}{1 + |z|}
+ * \f]
+ */
+BEGIN_DEFINE_ACTIVATION(softsign)
+private:
+MatrixPtr denominator_;
+
+Error __must_check forward(Argument& act) {
+  size_t height = act.value->getHeight();
+  size_t width = act.value->getWidth();
+  Matrix::resizeOrCreate(
+      denominator_, height, width, false, useGpu(act.deviceId));
+  denominator_->assign(*act.value);
+  denominator_->abs2();
+  denominator_->add(1.);
+
+  act.value->dotDiv(*act.value, *denominator_);
+  return Error();
+}
+
+Error __must_check backward(Argument& act) {
+  denominator_->square2();
+  denominator_->scalarDiv(*denominator_, 1.);
+  act.grad->dotMul(*act.grad, *denominator_);
+  return Error();
+}
+END_DEFINE_ACTIVATION(softsign)
+
 /**
  * @brief Relu Activation.
  * forward. y = max(0, z)

python/paddle/trainer_config_helpers/activations.py

@@ -17,7 +17,8 @@ __all__ = [
     "IdentityActivation", "LinearActivation", 'SequenceSoftmaxActivation',
     'ExpActivation', "ReluActivation", "BReluActivation", "SoftReluActivation",
     "STanhActivation", "AbsActivation", "SquareActivation", "BaseActivation",
-    "LogActivation", "SqrtActivation", "ReciprocalActivation"
+    "LogActivation", "SqrtActivation", "ReciprocalActivation",
+    "SoftSignActivation"
 ]
 
 
@@ -243,8 +244,20 @@ class ReciprocalActivation(BaseActivation):
     Reciprocal Activation.
 
     .. math::
-       f(z) = 1/z
+       f(z)=\\frac{1}{z}
     """
 
     def __init__(self):
         BaseActivation.__init__(self, 'reciprocal', False)
+
+
+class SoftSignActivation(BaseActivation):
+    """
+    SoftSign Activation.
+
+    .. math::
+       f(z)=\\frac{1}{1 + |z|}
+    """
+
+    def __init__(self):
+        BaseActivation.__init__(self, 'softsign', False)