class_docstr: A hash that generates group key and instance key for AllReduce.
docstr: Init the collective key.
docstr_tok: ["Init","the","collective","key","."]
code: def __init__(self, group_key_start=1):
      """Init the collective key."""
      self._group_key = group_key_start
      self._group_key_dict = {}
      self._instance_key_dict = {}
  
file_path: autodist/autodist/kernel/synchronization/collective_key.py
imports: import threading
  import hashlib
  from autodist.const import MAX_INT32
label_desc: Use image node_id 1 to create a new CollectiveKey object  with example: obj = CollectiveKey(group_key_start)