fix:In pytorch 0.4 accuracy always zero

[elliottzheng]

Running test in pytorch 0.4 , the accurary is always zero, because we need to transform zero dimensional tensor to python number by .item(), so I made a small fix.

[fxia22]

thanks a lot, by the way, are you able to use nndistance module in 0.4? Some users report that there is out of memory error.