Replaces the non-existing _param_init_net net by raising an exception.
Replaces the non-existing _param_init_net net by raising an exception.

Summary:
The _param_init_net does not exist. All the other places reference
param_init_net instead. So far no one has encountered any problem
because all the passed params are BlobReferences. This diff makes
this assumption explicit.

Reviewed By: azzolini

Differential Revision: D4922930

fbshipit-source-id: e6dbd7a29ea640b7e62fcfec7ced3cc7d149f872

sf-wind/caffe2,pietern/caffe2,bwasti/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,caffe2/caffe2,Yangqing/caffe2,davinwang/caffe2,pietern/caffe2,pietern/caffe2,sf-wind/caffe2,xzturn/caffe2,bwasti/caffe2,xzturn/caffe2,davinwang/caffe2,Yangqing/caffe2,bwasti/caffe2,sf-wind/caffe2,bwasti/caffe2,pietern/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,Yangqing/caffe2,davinwang/caffe2,sf-wind/caffe2