Trial's end time is less than start time in auto gbdt example

[lvybriage]

Short summary about the issue/question: trial's duration < 0

Brief what process you are following:

1. I run a example: nni/examples/trials/auto-gbdt/config.yml
2. In detail page's table, I find that some trial's duration is less than 0.
3. I checked the api (/api/v1/nni/trial-jobs), find those trials' end-time is less start-time.

How to reproduce it: run this example (nni/examples/trials/auto-gbdt/config.yml), maxTrials = 500

nni Environment: local

• nni version: v0.8
• nni mode(local|pai|remote): --
• OS: --
• python version: --
• is conda or virtualenv used?: --
• is running in docker?: --

need to update document(yes/no): no

Anything else we need to know: --

[lvybriage]

duplicated issue with #452