Add Sigmoid Cross Entropy Loss

[tanmayb123]

a.k.a. "binary_crossentropy" from Keras

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[tanmayb123]

I signed it!

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[saeta]

This is super great @tanmayb123 ! Thank you very much. In order to help raise the bar for quality, I've asked if you could possibly add a documentation comment to this new loss function. Let me know if that's confusing.

I'm going to mark these changes as approved, however, so you don't block on my subsequent review.

Have a great weekend!

-Brennan

[tanmayb123]

Thanks @saeta :)

I've added documentation comments for each loss function.

[rxwei]

Thanks Tanmay!