<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_flat_core_sizes(cores):
 """Obtains the list flattened output sizes of a list of cores. Args: cores: list of cores to get the shapes from. Returns: List of lists that, for each core, contains the list of its output dimensions. """

  core_sizes_lists = []
  for core in cores:
    flat_output_size = nest.flatten(core.output_size)
    core_sizes_lists.append(
        [tf.TensorShape(size).as_list() for size in flat_output_size])
  return core_sizes_lists