code: def batch_size(self):
      return len(self._data[0])
  
imports: import nvidia.dali.fn
  from nvidia.dali.pipeline import Pipeline
  from nose.tools import nottest
  import numpy
  from test_utils import np_type_to_dali
  from nose_utils import assert_raises
label_desc: Use image node_id 3 for calling the Batch obj's underlying member method code with example usage: obj.batch_size() and returns: len