[Bugfix][TF] reset graph after getting tag of savedmodel (#4055)
[Bugfix][TF] reset graph after getting tag of savedmodel (#4055)

@zhiics @icemelon9
sxjscience/tvm,dmlc/tvm,dmlc/tvm,dmlc/tvm,tqchen/tvm,tqchen/tvm,tqchen/tvm,Laurawly/tvm-1,sxjscience/tvm,dmlc/tvm,dmlc/tvm,tqchen/tvm,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,sxjscience/tvm,sxjscience/tvm,tqchen/tvm,Laurawly/tvm-1,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,Laurawly/tvm-1,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,sxjscience/tvm,sxjscience/tvm,dmlc/tvm,sxjscience/tvm,tqchen/tvm