Created by: reyoung

There is not prefetch method in class GradientMachine defined in PaddleAPI.h However, we need to add this method, which related to #364 (closed). In particular, we need to use it with sparse_udpate and sparse_remote_update.

When used with sparse update, prefetch is supposed to be invoked before forward/backward. sparse_update through SWIG cannot be performed without prefetch.

Also prefetch should be added to the monkey patch method forwardTest when sparse update is enabled.

The whole architecture of multi-language support is described by issue #258 (closed) .