docs: def add_variable(var, restore=True):
    """Adds a variable to the MODEL_VARIABLES collection.
  
      Optionally it will add the variable to  the VARIABLES_TO_RESTORE collection.
    Args:
      var: a variable.
      restore: whether the variable should be added to the
        VARIABLES_TO_RESTORE collection.
  
    """
    collections = [MODEL_VARIABLES]
    if restore:
      collections.append(VARIABLES_TO_RESTORE)
    for collection in collections:
      if var not in tf.get_collection(collection):
        tf.add_to_collection(collection, var)