code: def nesterov(step_size: Schedule, mass: float):
      """Construct optimizer triple for SGD with Nesterov momentum.
  
      Args:
        step_size: positive scalar, or a callable representing a step size schedule
          that maps the iteration index to a positive scalar.
        mass: positive scalar representing the momentum coefficient.
  
      Returns:
        An (init_fun, update_fun, get_params) triple.
      """
      step_size = make_schedule(step_size)
  
      def init(x0):
          v0 = jnp.zeros_like(x0)
          return x0, v0
  
      def update(i, g, state):
          x, velocity = state
          velocity = mass * velocity + g
          x = x - step_size(i) * (mass * velocity + g)
          return x, velocity
  
      def get_params(state):
          x, _ = state
          return x
  
      return init, update, get_params
  
imports: from __future__ import annotations
  from typing import Any, Callable, NamedTuple
  from collections import namedtuple
  import functools
  from functools import partial
  import jax.numpy
  from jax._src.util import safe_zip, safe_map, unzip2
  from jax import tree_util
  from jax.tree_util import tree_map, tree_flatten, tree_unflatten, register_pytree_node
label_desc: Use image node_id 4 for calling a global function with example usage: nesterov(step_size, mass) and returns: init,  update,  get_params, x0,  v0, x,  velocity, x