Fix the sorting kernels used in the CenterNet pre and post-processing.

This commit moves the kernels in the correct sub-directory.
It creates new header file for Thrust kernels. It splits the
kernels into two files: 'normalize.cu' contains CenterNet
pre-processing operations, 'postprocessing.cu' contains the
CenterNet post-processing operations.

Signed-off-by: Davide Sapienza <sapienza.dav@gmail.com>

src/CenternetDetection.cpp

@@ -1,10 +1,8 @@
 #include "CenternetDetection.h"
 
-#include "CenternetDetection.h"
 
 namespace tk { namespace dnn {
 
-
 bool CenternetDetection::init(const std::string& tensor_path, const int n_classes)
 {
     std::cout<<(tensor_path).c_str()<<"\n";