Add AtleastNOperands trait and update tf-ops test

* TensorFlow Control Flow supports variadic number of control inputs, add variant of NOperands to support at least N operands;
* Update example:
  - All ops will produce control output;
  - Use tf$name for mapping back to TensorFlow (a op can have a name as well as an attribute _name, using tf$name disambiguates that);

PiperOrigin-RevId: 206621280