code: def ref_stack(input_batches, axis):
      N = len(input_batches[0])
      out = []
      for i in range(N):
          inputs = [x[i] for x in input_batches]
          out.append(np.stack(inputs, axis=axis))
      return out
  
file_path: DALI/dali/test/python/operator_1/test_join.py
imports: import nvidia.dali
  import nvidia.dali.fn
  import numpy
  import math
  from test_utils import check_batch
label_desc: Use image node_id 6 for calling a global function with example usage: ref_stack(input_batches, axis) and returns: out