optional summary

[AndreasMadsen]

This makes it possible to disable summaries in a sg_context scope using with sg_context(summary=False).

The motivation for this is that I otherwise end up having 1000+ summaries, it takes a long time load in tensorboard and I can't open the taps anyway as the browser will just freeze.

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In order to implement this, a sg_summary_func decorator similar to sg_sugar_func and sg_layer_func was added. This decorator simplifies the summary implementations quite a lot and allows sg_context to affect the summary functions.

Unfortunately, this introduces a two small API change.

• if prefix=None is set the default prefix now is used, where it before removed the prefix. To remove the prefix prefix='' should now be used. I hope this is okay, to my mind it actually makes more sense.
• The required gradient argument in sg_summary_gradient must now be provided as a keyword argument. This is similar to sg_ce and the other loss functions.

[buriburisuri]

@AndreasMadsen Thanks for your contribution. I've updated my repo as your guide. Thank you again.