Pytorch README outdated

[jhrobres]

Instructions in link [1] are outdated, the branch (gh/xiaobingsuper/18/orig) to which one is supposed to checkout does not exist anymore. Thank you so much in advance and please, let me know if the branch is already merged into upstream.

[1] https://github.com/intel/optimized-models/tree/master/pytorch/ResNext101_32x4d#prepare-your-running-environment