add selu activation

Author: jalvesz

src/stdlib_specialfunctions.fypp

@@ -163,6 +163,32 @@ module stdlib_specialfunctions
     end interface
     public :: gelu_approx_grad
 
+    interface selu
+        !! Version: experimental
+        !!
+        !! Scaled Exponential Linear Unit
+        #:for rk, rt in REAL_KINDS_TYPES
+        elemental module function selu_${rk}$( x ) result( y )
+            ${rt}$, intent(in) :: x
+            ${rt}$ :: y
+        end function
+        #:endfor
+    end interface
+    public :: selu
+
+    interface selu_grad
+        !! Version: experimental
+        !!
+        !! Scaled Exponential Linear Unit
+        #:for rk, rt in REAL_KINDS_TYPES
+        elemental module function selu_grad_${rk}$( x ) result( y )
+            ${rt}$, intent(in) :: x
+            ${rt}$ :: y
+        end function
+        #:endfor
+    end interface
+    public :: selu_grad
+
     interface sigmoid
         !! Version: experimental
         !!

src/stdlib_specialfunctions_activations.fypp

@@ -112,6 +112,36 @@ end function
 
 #:endfor
 
+!==================================================
+! Scaled Exponential Linear Unit (SELU)
+!==================================================
+#:for rk, rt in REAL_KINDS_TYPES
+elemental module function selu_${rk}$( x ) result( y )
+    ${rt}$, intent(in) :: x
+    ${rt}$ :: y
+    ${rt}$, parameter :: scale = 1.0507009873554804934193349852946_${rk}$
+    ${rt}$, parameter :: alpha = 1.6732632423543772848170429916717_${rk}$
+    if(x > 0._${rk}$)then
+        y = scale * x
+    else
+        y = scale * (alpha * exp(x) - alpha)
+    end if
+end function
+
+elemental module function selu_grad_${rk}$( x ) result( y )
+    ${rt}$, intent(in) :: x
+    ${rt}$ :: y
+    ${rt}$, parameter :: scale = 1.0507009873554804934193349852946_${rk}$
+    ${rt}$, parameter :: alpha = 1.6732632423543772848170429916717_${rk}$
+    if(x > 0._${rk}$)then
+        y = scale
+    else
+        y = scale * alpha * exp(x)
+    end if
+end function
+
+#:endfor
+
 !==================================================
 ! Sigmoid
 !==================================================