class_docstr: The Adder interface.
  
  An adder packs together data to send to the replay buffer, and potentially
  performs some reduction/transformation to this data in the process.
  
  All adders will use this API. Below is an illustrative example of how they
  are intended to be used in a typical RL run-loop. We assume that the
  environment conforms to the dm_env environment API.
  
  ```python
  # Reset the environment and add the first observation.
  timestep = env.reset()
  adder.add_first(timestep.observation)
  
  while not timestep.last():
    # Generate an action from the policy and step the environment.
    action = my_policy(timestep)
    timestep = env.step(action)
  
    # Add the action and the resulting timestep.
    adder.add(action, next_timestep=timestep)
  ```
  
  Note that for all adders, the `add()` method expects an action taken and the
  *resulting* timestep observed after taking this action. Note that this
  timestep is named `next_timestep` precisely to emphasize this point.
docstr: Defines the interface for an adder's `add_first` method.
  
  We expect this to be called at the beginning of each episode and it will
  start a trajectory to be added to replay with an initial observation.
  
  Args:
    timestep: a dm_env TimeStep corresponding to the first step.
docstr_tok: ["Defines","the","interface","for","an","adder","'s","`","add_first","`","method",".","We","expect","this","to","be","called","at","the","beginning","of","each","episode","and","it","will","start","a","trajectory","to","be","added","to","replay","with","an","initial","observation",".","Args",":","timestep",":","a","dm_env","TimeStep","corresponding","to","the","first","step","."]
code: def add_first(self, timestep: dm_env.TimeStep):
      """Defines the interface for an adder's `add_first` method.
  
      We expect this to be called at the beginning of each episode and it will
      start a trajectory to be added to replay with an initial observation.
  
      Args:
        timestep: a dm_env TimeStep corresponding to the first step.
      """
  
imports: import abc
  from acme import types
  import dm_env
label_desc: Use image node_id 1 for calling the Adder obj's underlying member method code with example usage: obj.add_first(timestep) without return types