Hi,
one way to solve it is to flatten the tensor, take the maximum index along dimension 0  and then unravel the index 171, either using numpy or your own logic (probably you will come up with something less clumsy  ):
rawmaxidx = mytensor.view(-1).min(0)[1]
idx = []
for adim in list(mytensor.size())[::-1]:
    idx.append(rawmaxidx%adim)
    rawmaxidx = rawmaxidx / adim
idx = torch.cat(idx)

(Note that pytorch / on LongTensor is similar to python2 / or python3 // for ints.
Best regards
Thomas