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()``.
code: def __init__(self, items: Sequence[ConfigItem] | None = None):
      # save the items in a dictionary with the `ConfigItem.id` as key
      self.items: dict[str, ConfigItem] = (
          {} if items is None else {i.get_id(): i for i in items}
      )
      self.resolved_content: dict[
          str, ConfigExpression | str | Any | None
      ] = {}
  
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 1 to create a new ReferenceResolver object  with example: obj = ReferenceResolver(items)