Fix regression in regression
# What does this PR do?

This PR fixes the regression introduced in #11012 for regression problems with only one label (like STS-B), see discussion on #11780. I checked both `run_glue` and `run_glue_no_trainer` on this branch and get the proper results for this task now.

Fixes #11780
Fixes #11583
Thank you for fixing the issue! 