[nvccWarnings] Fix warnings seen with dtests on nvcc path

tests/src/runtimeApi/memory/hipMemcpy.cpp

@@ -192,8 +192,6 @@ HostMemory<T>::~HostMemory ()
         free(A_hh);
         free(B_hh);
     }
-    T *A_hh = NULL;
-    T *B_hh = NULL;
 
 };
 

tests/src/runtimeApi/memory/hipMemcpyPeer.cpp

@@ -32,7 +32,6 @@ THE SOFTWARE.
 
 int main()
 {
-    hipDevice_t device;
     size_t Nbytes = N*sizeof(int);
     int numDevices = 0;
     int *A_d, *B_d, *C_d, *X_d, *Y_d, *Z_d;

tests/src/runtimeApi/memory/p2p_copy_coherency.cpp

@@ -188,7 +188,6 @@ int main(int argc, char *argv[])
 {
     HipTest::parseStandardArguments(argc, argv, true);
 
-    int numElements = N;
 
     int dev0 = 0;
     int dev1 = 1;

tests/src/runtimeApi/stream/hipStreamAddCallback.cpp

@@ -27,7 +27,10 @@ THE SOFTWARE.
 
 #include "hip/hip_runtime.h"
 #include "test_common.h"
+
+#ifdef __HIP_PLATFORM_HCC__ 
 #define HIPRT_CB
+#endif
 
 class CallbackClass
 {