Merge dataset when the dimension of y is 1

[advikavs]

Description

Fix #3130

Dataset merge does not work when the dimension of y is 1. We fix this by checking the dimension and reshaping accordingly.

Type of change

• [ x] Bug fix (non-breaking change which fixes an issue)

[rbharath]

Since we merged in the 4 space yapf, we may need to do a preliminary PR running yapf on datasets.py before we can merge this PR in.

We also look to have a lot of mypy errors (unrelated to this PR but in general)

[arunppsg]

Since we merged in the 4 space yapf, we may need to do a preliminary PR running yapf on datasets.py before we can merge this PR in.

yapf for datasets.py is done in #3166 . Merging this in.