code: def batch_mean(batch, axes):
      mean = None
      for x in batch:
          tmp = np.sum(x, axis=axes, keepdims=True)
          if mean is None:
              mean = tmp
          else:
              mean += tmp
      return mean / batch_reduced_vol(batch, axes)
  
imports: from nvidia.dali.pipeline import Pipeline
  from nvidia.dali import backend
  import nvidia.dali.ops
  import numpy
  from test_utils import dali_type
  from nvidia.dali import fn, pipeline_def, types
  from nose2.tools import params
label_desc: Use image node_id 3 for calling a global function with example usage: batch_mean(batch, axes) and returns: unknown