[VTA][Hotfix] Avoiding error when environment variable is not set (#3497)
[VTA][Hotfix] Avoiding error when environment variable is not set (#3497)

* avoid error when env var is not set

* extra content

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