docstr: Call the source callback and feed the results to the ExternalSource nodes in `pipeline`.
  Used for the sequential ExternalSource variant.
docstr_tok: ["Call","the","source","callback","and","feed","the","results","to","the","ExternalSource","nodes","in","`","pipeline","`",".","Used","for","the","sequential","ExternalSource","variant","."]
code: def get_batch(self, pipeline, batch_size, epoch_idx):
      """Call the source callback and feed the results to the ExternalSource nodes in `pipeline`.
      Used for the sequential ExternalSource variant."""
      try:
          if self.batch:
              callback_out = self.callback(
                  *self.callback_args(None, epoch_idx)
              )
          else:
              callback_out = [
                  self.callback(*self.callback_args(i, epoch_idx))
                  for i in range(batch_size)
              ]
          self.current_sample += batch_size
          self.current_iter += 1
      except StopIteration:
          self.reset_indices()
          raise
      return _ExternalDataBatch(
          self, pipeline, callback_out, batch_size
      )
  
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 9 for calling the _ExternalSourceGroup obj's underlying member method code with example usage: obj.get_batch(pipeline, batch_size, epoch_idx) and returns: _ExternalDataBatch