Change unsigned long to unsigned long long in gloo/cuda_allreduce_ring_chunked.cc and gloo/allreduce_ring_chunked.h

[gunandrose4u]

Code review comment from Orvid in PR #264

Just make this explicitly an unsigned long long. long is 64-bit on linux, but 32-bit on windows, so it's far better to make this explicit. (I would recommend searching through gloo for any references to bare long and fixing them, otherwise you're likely to run into issues later)

Issues:
Simply change unsigned long to unsigned long long will leads test code compile error. Need investigate and fix conflicts in test code introduced by this change.