Add missing C++ header in example

Author: 9prady9

examples/autograd.cpp

@@ -10,6 +10,8 @@
 #include <af/autograd.h>
 #include <af/nn.h>
 
+#include <iostream>
+
 #define VERIFY(VAL) do {                                    \
         auto res = af::allTrue<bool>(af::abs(VAL) < 1E-5);  \
         printf("%s:%d %s\n", __FUNCTION__, __LINE__,        \