Add customizable dropout layer with compile-time rate specification (davisking#3000)

* Add customizable dropout layer with compile-time rate specification

* Update to the name of the new dropout rate customisation class

* Fix: Replace float template parameter with int for C++17 compatibility

* Update dlib/dnn/layers_abstract.h

---------

Co-authored-by: Davis E. King <davis685@gmail.com>

Author: Cydral

dlib/dnn/layers.h

@@ -2134,6 +2134,24 @@ namespace dlib
     template <typename SUBNET>
     using dropout = add_layer<dropout_, SUBNET>;
 
+// ----------------------------------------------------------------------------------------
+
+    template <int DROP_RATE_PERCENT>
+    class dropout_rate_ : public dropout_
+    {
+    public:
+        explicit dropout_rate_() : dropout_(static_cast<float>(DROP_RATE_PERCENT) / 100.0f)
+        {
+            static_assert(DROP_RATE_PERCENT >= 0 && DROP_RATE_PERCENT <= 100,
+                "DROP_RATE_PERCENT must be between 0 and 100, inclusive.");
+        }
+    };
+    
+    template <int DROP_RATE, typename SUBNET>
+    using dropout_rate = add_layer<dropout_rate_<DROP_RATE>, SUBNET>;
+    template <typename SUBNET>
+    using dropout_10 = add_layer<dropout_rate_<10>, SUBNET>;
+
 // ----------------------------------------------------------------------------------------
 
     class multiply_

dlib/dnn/layers_abstract.h

@@ -1433,6 +1433,41 @@ namespace dlib
     template <typename SUBNET>
     using dropout = add_layer<dropout_, SUBNET>;
 
+// ----------------------------------------------------------------------------------------
+
+    template <int DROP_RATE_PERCENT>
+    class dropout_rate_ : public dropout_
+    {
+        /*!
+            WHAT THIS OBJECT REPRESENTS
+                This object represents a customizable dropout layer that inherits from
+                the dropout_ class. It allows specifying the dropout rate at compile-time,
+                which is particularly useful for deep networks with many layers where it
+                might be cumbersome to explicitly modify the dropout rate for each layer
+                individually.
+
+                The main advantage of this layer is that it offers the possibility to specify
+                the dropout rate at the moment of network construction, providing more
+                flexibility and clarity in the network architecture definition.
+
+            TEMPLATE PARAMETERS
+                - DROP_RATE_PERCENT: A int value between 0 and 100 that specifies the dropout rate.
+                  This value is set at compile-time and cannot be changed during runtime.
+        !*/
+
+    public:
+        explicit dropout_rate_();
+        /*!
+            ensures
+                - Constructs a dropout layer with a dropout rate of DROP_RATE.
+                - Calls the base class constructor dropout_(DROP_RATE).
+        !*/
+    };
+
+    template <int DROP_RATE, typename SUBNET>
+    using dropout_rate = add_layer<dropout_rate_<DROP_RATE>, SUBNET>;
+    template <typename SUBNET>
+    using dropout_10 = add_layer<dropout_rate_<10>, SUBNET>;
 // ----------------------------------------------------------------------------------------
 
     class multiply_