class_docstr: Simple wrapper around shared memory chunks. Adds mem_chunk_id used
  to identify chunks in the communication between parent and worker process.
code: def __init__(
      self, shm_chunk_id, capacity, shm_chunk: shared_mem.SharedMem
  ):
      self.shm_chunk_id = shm_chunk_id
      self.capacity = capacity
      self._shm_chunk = shm_chunk
  
imports: import os
  from nvidia.dali._multiproc import shared_mem
  from nvidia.dali._multiproc.messages import ShmMessageDesc
  from nvidia.dali._utils.external_source_impl import assert_cpu_sample_data_type, sample_to_numpy
  import pickle
label_desc: Use image node_id 1 to create a new BufShmChunk object  with example: obj = BufShmChunk(shm_chunk_id, capacity, shm_chunk)