Add ProxyNCA++ loss

[reactivetype]

"ProxyNCA++ adds enhancements to ProxyNCA and achieves a 22.9 percentage point average improvement of Recall@1 across four different zero-shot retrieval datasets compared to the original ProxyNCA algorithm.

Furthermore, we achieve state-of-the-art results on the CUB200, Cars196, Sop, and InShop datasets, achieving Recall@1 scores of 72.2, 90.1, 81.4, and 90.9, respectively."

https://arxiv.org/abs/2004.01113