add logsigmoid op

[beverlylytle]

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• Did you write any new necessary tests?

What does this PR do?

Add support for the logsigmoid activation function.

PR review

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Did you have fun?

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[mruberry]

Exciting!

I made some suggestions for your review, @beverlylytle. I hope you find looking into implementation speed and memory use as interesting as I do!

[mruberry]

Rad!

I filed #1555 for avoiding the creation of the extra CUDA tensor by adding a custom autograd formula for logsigmoid that would only apply when using the PyTorch executor