bugfix for multi-molecules in scaffold split

[hesther]

Another bug fix for multi-molecule input. This is analogous to the changes that were made in 2c98bf7 (checking if multi-molecule input is used, raise an exception, else flatten the list of molecules).

[kevingreenman]

Should a similar change be made in chemprop/data/scaffold.py around line 71?

[hesther]

This change was already made to chemprop/data/scaffold.py in commit 2c98bf7. On (current) line 70+71, we check whether the number of molecules is larger than 1, and in line 79 we flatten the list of lists in chemprop/data/scaffold.py. So yes, it absolutely also affects chemprop/data/scaffold.py, but it was already fixed, and somehow scripts/create_crossval_splits.py was forgotten at that time.

[mliu49]

Looks good to me!