input: Write a function to generate nested loops that print out the elements of given lists in a new line. The function should take a list of lists as input and return the nested loops. If the input list is empty, it should print an appropriate message.
  assert nested_loop_print([[1,2,3], [4,5,6], [7,8,9]])=='''for x in [1,2,3]:\n  for y in [4,5,6]:\n      for z in [7,8,9]:\n          print(x,y,z)'''
  assert nested_loop_print([])=='''No input provided. Please provide a list of lists.'''
output: def nested_loop_print(lst):
      if not lst:
          return "No input provided. Please provide a list of lists."
      loops = []
      for i, sublist in enumerate(lst):
          loops.append(f"for var{i} in {sublist}:")
      loops.append("    print(" + ", ".join(f"var{i}" for i in range(len(lst))) + ")")
      return "\n".join(loops)