caffe2: relax PartitionOp constraints
caffe2: relax PartitionOp constraints

Summary: We actually copy items inside, so no need to limit this to POD types.

Reviewed By: dzhulgakov

Differential Revision: D4768652

fbshipit-source-id: 98f71b78a7c1dd4a2a2e1bff096d6bf63a0c8f50

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