Static graph computation

[23pointsNorth]

Parts of the models that are defined here should benefit from the fact they are static graphs.

I have not seen anywhere in the repository the @static_code tag along the __call__ methods for instance in ssd, fast_rcnn.

Am I right to expect that making the graphs static where I have marked would speed up computation?

[yuyu2172]

In principle, it should speed up computation in some situations. We have not yet tested that feature for our models.