Solved by Shisho_Sama in post #3 
              
              
                OK. it seems it was caused becasue of a mistmatched versions between the installed torch (1.5.1) and using the latest libtorch (nightly build). 
I had previously reverted back to 1.5.1 and forgot that the libtorch was from the latest nightly build. so this was casued. 
removing and reinstalling both…