Suggestion for improving pairwise distance calculation

[bdalal]

Hi,
Thanks for the great work on this library!
I have a small suggestion to improve the pairwise distance calculation

https://github.com/PyRetri/PyRetri/blob/master/pyretri/index/re_ranker/re_ranker_impl/k_reciprocal.py#L35-L51
I see this function being used in 3 places - kreciprocal, knn and query expansion.

This function can simple be replaced by 1 line - torch.cdist(query_fea, gallery_fea)
PyTorch's official docs don't list this function for 1.2 but it is supported (I've used it).

This change will reduce duplication and make the code more readable.

Thanks!

[hby96]

Thanks for you suggestions! I will fix it in the next vesion.