Add _train_execs to model so that we can get reference to the training executor from the outside.
Add _train_execs to model so that we can get reference to the training executor from the outside.

sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet