[AutoTVM] select model with the most tuned schedules (#4404)
[AutoTVM] select model with the most tuned schedules (#4404)

* select model with the most tuned schedules

* change detect empty map method

* modify model description for load_reference_log

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