Failed trials show a huge negative duration on macOS

[ultmaster]

Short summary about the issue/question:

Duration shows a huge negative number when the trial gets failed.

Brief what process you are following:

How to reproduce it: Install nii using pip and run mnist example.

nni Environment:

• nni version: 0.9.1.1
• nni mode(local|pai|remote): local
• OS: macOS Mojave
• python version: Python 3.6
• is conda or virtualenv used?: virtualenv
• is running in docker?: no

need to update document(yes/no): no

Anything else we need to know:

I'm pretty sure it's a problem related to Mac or macOS.

[ultmaster]

https://superuser.com/questions/599072/how-to-get-bash-execution-time-in-milliseconds-under-mac-os-x

The problem is date +%s%3N is not supported in macOS. So needs an extra if for platform darwin.