code: public DifferentialFunction getFunctionById(@NonNull String id) {
          if (!ops.containsKey(id)) {
              throw new ND4JIllegalStateException("No function with id " + id + " found!");
          }
          return ops.get(id).getOp();
      }
docs: Get the function by the {@link DifferentialFunction#getOwnName()}
  
  @param id the id of the function
  @return the function for the given id if it exists