Fixes several issues with gradients and some other bugs

rather than having an error

was iterable

wrt

given bad input

Lop wasn't complaining because it had no input validation. Razvan wrote
these tests and doesn't remember what they were meant to do, so I am
removing them.

added some input validation to Lop

by other problems

consistently

their gradient to 0

Corrected a typo in docstring