code: def prefetch(self, pool, context_i, batch_size, epoch_idx):
      # NOTE We can't schedule more than what's on top of pipeline's prefetch queue, as the
      # entires in the pipeline are zero-copy and cannot be overwritten.
      context = pool.contexts[context_i]
      while (
          context.scheduled_ahead < self.prefetch_queue_depth
          and self.schedule_batch(
              pool,
              context_i,
              context.scheduled_ahead,
              batch_size,
              epoch_idx,
          )
      ):
          pass
  
imports: from nvidia.dali import backend
  from nvidia.dali import tensors
  from nvidia.dali import types
  from nvidia.dali._multiproc.messages import TaskArgs, SampleRange
  import nvidia.dali.types
  from nvidia.dali._utils.external_source_impl import get_callback_from_source, accepted_arg_count, SourceKind
label_desc: Use image node_id 6 for calling the _ExternalSourceGroup obj's underlying member method code with example usage: obj.prefetch(pool, context_i, batch_size, epoch_idx) without return types