[caffe2][memonger] Add support for distributed inference predict nets in DAG memonger
Summary:
Distributed Inference splits a predict net into multiple parts, part0 being the main part which contains ops to make remote calls to other parts. part0 predict net may contain AsyncIf ops to optimize rpc call usage. AsyncIf ops have internal nets which may refer to memongered blobs. This change handles AsyncIf ops to update internal nets to refer to memongered blobs. Here is one reference part0 predict net with AsyncIf ops: https://www.internalfb.com/intern/paste/P145812115/
As part of this change, I am also updating dag memonger traversal to always start from root op, i.e. ops with 0 in degree. Earlier logic will start traversing ops based on input head blobs and if one of the head inputs is getting used in a non-root op which gets visited before its parent, the traversal will throwing assertion error here: https://fburl.com/diffusion/ob110s9z . Almost for all the distributed inference part0 nets, it was throwing this assertion error.
Reviewed By: hlu1
Differential Revision: D24346771
fbshipit-source-id: ad2dd2e63f3e822ad172682f6d63f8474492255d
