Added a warning when output doesn't seem to depend on input.
Added a warning when output doesn't seem to depend on input.

Technically, this should be allowable behavior but in practice it almost always
indicates a bug.

HIPS/autograd,melgor/autograd,barak/autograd,t1m0thy/autograd,hips/autograd,hips/autograd,kcarnold/autograd,jackkamm/autograd,HIPS/autograd,FighterLYL/autograd