class_docstr: Utility class to manage a set of ``ConfigItem`` and resolve the references between them.
  
  This class maintains a set of ``ConfigItem`` objects and their associated IDs.
  The IDs must be unique within this set. A string in ``ConfigItem``
  starting with ``@`` will be treated as a reference to other ``ConfigItem`` objects by ID.
  Since ``ConfigItem`` may have a nested dictionary or list structure,
  the reference string may also contain the separator ``::`` to refer to a substructure by
  key indexing for a dictionary or integer indexing for a list.
  
  In this class, resolving references is essentially substitution of the reference strings with the
  corresponding python objects. A typical workflow of resolving references is as follows:
  
      - Add multiple ``ConfigItem`` objects to the ``ReferenceResolver`` by ``add_item()``.
      - Call ``get_resolved_content()`` to automatically resolve the references. This is done (recursively) by:
          - Convert the items to objects, for those do not have references to other items.
              - If it is instantiable, instantiate it and cache the class instance in ``resolved_content``.
              - If it is an expression, evaluate it and save the value in ``resolved_content``.
          - Substitute the reference strings with the corresponding objects.
  
  Args:
      items: ``ConfigItem``s to resolve, this could be added later with ``add_item()``.
docstr: Split the id string into a list of strings by `cls.sep`.
  
  Args:
      id: id string to be split.
      last: whether to split the rightmost part of the id. default is False (split all parts).
docstr_tok: ["Split","the","id","string","into","a","list","of","strings","by","`","cls.sep","`",".","Args",":","id",":","id","string","to","be","split",".","last",":","whether","to","split","the","rightmost","part","of","the","id",".","default","is","False","(","split","all","parts",")","."]
code: def split_id(cls, id: str | int, last: bool = False) -> list[str]:
      """
      Split the id string into a list of strings by `cls.sep`.
  
      Args:
          id: id string to be split.
          last: whether to split the rightmost part of the id. default is False (split all parts).
      """
      if not last:
          return cls.normalize_id(id).split(cls.sep)
      res = cls.normalize_id(id).rsplit(cls.sep, 1)
      return ["".join(res[:-1]), res[-1]]
  
imports: from __future__ import annotations
  import re
  import warnings
  from collections.abc import Sequence
  from typing import Any, Iterator
  from monai.bundle.config_item import ConfigComponent, ConfigExpression, ConfigItem
  from monai.bundle.utils import ID_REF_KEY, ID_SEP_KEY
  from monai.utils import allow_missing_reference, look_up_option
label_desc: Use image node_id 9 for calling the ReferenceResolver obj's underlying member method code with example usage: obj.split_id(cls, id, last) and returns: list, cls