[VTA] cannt import vta.testing from vta

Hi, everyone. I got stuck when I ran example code from Deploy Pretrained Vision Model from MxNet on VTA — tvm 0.11.dev0 documentation

I install VTA successfully and this testing cmd
python <tvm root>/vta/tests/python/integration/test_benchmark_topi_conv2d.py works well.

However, something went wrong when I type this.
import vta.testing from vta

But I’m sure that I have already ran this cmd
export PYTHONPATH=/path/to/vta/python:${PYTHONPATH}

someone has an idea what is wrong?