better loading of pretrain model

[haruishi43]

• load pretrain model via torch.utils.model_zoo instead of having to create a pretrain model destination (weights/pre_train).
• fixes bugs with bn.num_batches_tracked parameter with the old code
• remove scipy.misc.imsave since it is deprecated and replaced it with imageio.imwrite which functions the same

[ghost]

All CLA requirements met.

[haruishi43]

i've tried with and without this revision, but I couldn't recreate the same results as in the paper. this revision didn't change the results too much I'll post some results on #9 since there's a lot of attention on results there

[haruishi43]

Couldn't recreate the results:

Evaluated with "feat_" features and "cosine" metric:
Mean AP: 33.2%
CMC Scores
  top-1          30.7%
  top-5          56.6%
  top-10         69.6%
Evaluated with "feat" features and "cosine" metric:
Mean AP: 21.4%
CMC Scores
  top-1          17.7%
  top-5          36.8%
  top-10         52.3%