rename the print delegate graph api

Summary: The current api name is a bit weird and user will need to call `print(print_delegated_graph(graph_module)))`. Add an api `format_delegated_graph` which is the same as the original `print_delegated_graph`. For the original `print_delegated_graph` api, it just print the string returned from `format_delegated_graph`

Reviewed By: lucylq

Differential Revision: D56711090

fbshipit-source-id: 18fbbe52a9ba6674f0c81460928f25692ba3a83c