declare type name for optional<TShape> (#429)
declare type name for optional<TShape> (#429)

* declare type for optional tshape

* add doc

* move code to another place

Huyuwei/tvm,Huyuwei/tvm,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,dmlc/tvm,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,tqchen/tvm,dmlc/tvm,dmlc/tvm,Huyuwei/tvm,tqchen/tvm,Huyuwei/tvm,tqchen/tvm,Huyuwei/tvm,dmlc/tvm,sxjscience/tvm,dmlc/tvm,Huyuwei/tvm,tqchen/tvm,dmlc/tvm,Huyuwei/tvm,sxjscience/tvm,sxjscience/tvm,Huyuwei/tvm,dmlc/tvm,sxjscience/tvm,sxjscience/tvm,Laurawly/tvm-1,Huyuwei/tvm,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,tqchen/tvm,sxjscience/tvm