change schema's __repr__() flat output to pprint style indented output
change schema's __repr__() flat output to pprint style indented output

Summary: as title. This is similar with python pprint utility for nested json data structure. It can be useful for checking schema during debugging.

Reviewed By: kittipatv

Differential Revision: D6710767

fbshipit-source-id: e450aa5477fa1ad4f93c4573f8108a2f49956da8

Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,caffe2/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2