relu special case with simple lambda

Author: Dobiasd

include/fdeep/layers/relu_layer.hpp

@@ -8,6 +8,8 @@
 
 #include "fdeep/layers/activation_layer.hpp"
 
+#include <algorithm>
+#include <limits>
 #include <string>
 
 namespace fdeep {
@@ -29,6 +31,13 @@ namespace internal {
     protected:
         tensor transform_input(const tensor& in_vol) const override
         {
+            if (negative_slope_ == static_cast<float_type>(0) &&
+                threshold_ == static_cast<float_type>(0) &&
+                max_value_ == std::numeric_limits<float_type>::max()) {
+                return transform_tensor([](float_type x) -> float_type {
+                    return std::max(static_cast<float_type>(0), x);
+                }, in_vol);
+            }
             return transform_tensor([&](float_type x) -> float_type {
                 if (x >= max_value_)
                     return max_value_;