Returns the fluent shape given its `param_types`.

def _param_types_to_shape(self, param_types: Optional[str]) -> Sequence[int]:
        '''Returns the fluent shape given its `param_types`.'''
        param_types = [] if param_types is None else param_types
        shape = tuple(self.object_table[ptype]['size'] for ptype in param_types)
        return shape