how to clean a stainless steel chocolate piping( with chocolate inside )?

Answer 1

run it under hot water until all chocolate melts

Answer 2

It depends on how long the piping is. You can use a pipe cleaner, you can soak it in a boiling pot of water (remove with tongs so you don't burn yourself!), you can run a long skewer through it with a paper towel (which has been soaked in hot water) wrapped around the end.
Good luck!