Loss Operations: Adds log loss

[manrajgrover]

This PR adds log loss to the API.

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This change is 

[dsmilkov]

Thanks Manraj! You'll need to resolve some merge conflicts and then I'll merge this in.

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[dsmilkov]

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Reviewed 3 of 3 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable