Removed useless constraint

Note that the code was written with f being a variable or a list of variable in mind (the documentation says so, and we even convert f into a list right after the assert).

Removed useless constraint
d0509f8e · Razvan Pascanu · 7b943a5b · d0509f8e
--- a/theano/gradient.py
+++ b/theano/gradient.py
@@ -347,10 +347,6 @@ def Lop(f, wrt, eval_points, consider_constant=None, warn_type=False,
    if consider_constant is None:
        consider_constant = []
-    if not isinstance(f, Variable):
-        raise TypeError(('In Lop(), cost argument should be '
-                        'a Variable.'), f)
    if type(eval_points) not in (list, tuple):
        eval_points = [eval_points]