class_docstr: Task for value evaluation.
code: def l1_loss(self):
      def loss(values, targets, weights):
          return jnp.sum(jnp.abs(values - targets) * weights) / jnp.sum(
              weights
          )
  
      return tl.Fn("L1Loss", loss)
  
file_path: trax/trax/rl/value_tasks.py
imports: import copy
  import numpy
  from trax import layers
  from trax.fastmath import numpy
  from trax.supervised import training
label_desc: Use image node_id 2 for calling the ValueEvalTask obj's underlying member method code with example usage: obj.l1_loss() and returns: tl, unknown