MetisTuner doesn't support maximize mode

[suiguoxin]

Short summary about the issue/question:
Metis Tuner doesn't support maximize mode, it converges to the minimum in a toy experiment even though optimize_mode is defined as "maximum".

Brief what process you are following:

How to reproduce it:
Run an experiment in "maximize" mode.

nni Environment:

• nni version: 0.8
• nni mode(local|pai|remote): local
• OS: linux
• python version: 3.7
• is conda or virtualenv used?: conda
• is running in docker?: no

need to update document(yes/no): no

Anything else we need to know:
Seems that OptimizeMode from nni.utils is not working.

[xuehui1991]

Fix in #1195 .