Sometimes, user install a dev/editable version of vllm. In this case, we
should make sure vllm-ascend works as well.
This PR add a new env `VLLM_VERSION`. It's used for developers who edit
vllm. In this case, developers should set thie env to make sure which
vllm version is installed and used.
Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>