What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind bug

What does this PR do / why do we need it:
dataset: decrease num_worker when mechine didn't has so much

Which issue(s) this PR fixes:

Fixes #https://github.com/mindspore-ai/mindspore/issues/134

Special notes for your reviewers: