[WIP] Nystrom sinkhorn

[tvayer]

Types of changes

Objective: implement the method in "Massively scalable Sinkhorn distances via the Nyström method, Jason Altschuler, Francis Bach, Alessandro Rudi, Jonathan Niles-Weed, NeurIPS 2019.
So far: added low_rank_kernel for Sinkhorn, Nystrom kernel for Nystrom Sinkhorn, empirical_sinkhorn_nystroem
Also

TODO

• Add loss for empirical_sinkhorn_nystroem
• Add documentation
• Add tests for empirical_sinkhorn_nystroem
• Add tests for kernel approximation using Nystrom
• Put in the API ot.solve ?
• Add example
• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• Update readme with refs
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 33 lines in your changes missing coverage. Please review.

Project coverage is 85.23%. Comparing base (99477be) to head (1bae4b9).

❗ There is a different number of reports uploaded between BASE (99477be) and HEAD (1bae4b9). Click for more details.

HEAD has 19 uploads less than BASE

Flag	BASE (99477be)	HEAD (1bae4b9)
	20	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #742       +/-   ##
===========================================
- Coverage   97.09%   85.23%   -11.87%     
===========================================
  Files         100      100               
  Lines       20432    20579      +147     
===========================================
- Hits        19839    17541     -2298     
- Misses        593     3038     +2445     

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