add  argument in sum method's docstring
add  argument in sum method's docstring

kris-singh/pgmpy,abinashpanda/pgmpy,ankurankan/pgmpy,sandeepkrjha/pgmpy,khalibartan/pgmpy,ankurankan/pgmpy,pgmpy/pgmpy,jhonatanoliveira/pgmpy,sandeepkrjha/pgmpy,pgmpy/pgmpy,khalibartan/pgmpy,kris-singh/pgmpy,jhonatanoliveira/pgmpy,abinashpanda/pgmpy