'Returns a generator object of symbol.
One can loop through the returned object list to get outputs.
Example usage:
>>> a = mx.sym.Variable(\'a\')
>>> b = mx.sym.Variable(\'b\')
>>> c = a+b
>>> d = mx.sym.Variable(\'d\')
>>> e = d+c
>>> out = e.get_children()
>>> out
<Symbol Grouped>
>>> for i in out:
...     i
<Symbol d>
<Symbol _plus0>'

def __iter__(self):

    return (self[i] for i in self.list_outputs())
