Set automatic TPU testing

[chenmoneygithub]

Because of the capacity limit, we cannot do TPU testing on every PR push. We have two options here: 1) type gcbrun/ manually to trigger TPU testing, which should happen after normal working hours. 2) Do a nightly testing, the downside of it is we cannot link the testing results back to github, so we need to manually check the GCP log.

I would actually prefer 1), since in that way we are still using Github as the single place to check testing log.

[chenmoneygithub]

I set up the TPU test following 1, let's see how it goes in the next a few weeks.