Lifted structure loss

[Lorenzobattistela]

This PR refers to Issue #102 and it is implementing the LSL described in the article https://arxiv.org/pdf/1511.06452.pdf

[google-cla]

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[Lorenzobattistela]

@owenvallis fixed the CLA problem, same changes from before.

[owenvallis]

Looks like the tests/losses/test_lifted_structure_loss.py just needs to be formatted and then the PR should be good to go.

[Lorenzobattistela]

@owenvallis Sorry took so long, I missed your comment in the notifications. I ran black black . --verbose --check in my branch locally, and i got this output:
Oh no! 💥 💔 💥 14 files would be reformatted, 144 files would be left unchanged.

But i commited only the change in test_lsl, since it was the file related to this PR. If it is needed to change other things, just tag me and I'll do it. Thanks!

[owenvallis]

Looks like there are some isort errors with the import formatting but that shouldn't block us here. I'll format the imports on my side.

Thanks again for implementing the loss!

[Lorenzobattistela]

Appreciate all the help, I'll keep contributing to similarity. Thanks