correctly deal with job retries on openpai

[QuanluZhang]

Short summary about the issue/question: a job on openpai may be retried. In the current version, nni is not aware of such event. This may induce potential issues, for example, an assessor may find such a trial's learning curve is strange, leading to incorrect behavior.

Brief what process you are following: normal

How to reproduce it: when a trial is retried on openpai

nni Environment:

• nni version: 0.5.2
• nni mode(local|pai|remote): pai
• OS: ubuntu
• python version: 3.5
• is conda or virtualenv used?: no
• is running in docker?: no

Anything else we need to know:
Related to #863 and #865.

[leelaylay]

I think it is an important bug need to be fixed. Related to #863 and #865.