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: Iterate over the sub-configs of the input config, the output `sub_id` uses `cls.sep` to denote substructure.
  
  Args:
      id: id string of the current input config.
      config: input config to be iterated.
docstr_tok: ["Iterate","over","the","sub-configs","of","the","input","config",",","the","output","`","sub_id","`","uses","`","cls.sep","`","to","denote","substructure",".","Args",":","id",":","id","string","of","the","current","input","config",".","config",":","input","config","to","be","iterated","."]
code: def iter_subconfigs(
      cls, id: str, config: Any
  ) -> Iterator[tuple[str, str, Any]]:
      """
      Iterate over the sub-configs of the input config, the output `sub_id` uses `cls.sep` to denote substructure.
  
      Args:
          id: id string of the current input config.
          config: input config to be iterated.
      """
      for k, v in (
          config.items()
          if isinstance(config, dict)
          else enumerate(config)
      ):
          sub_id = f"{id}{cls.sep}{k}" if id != "" else f"{k}"
          yield k, sub_id, v
  
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 10 for calling the ReferenceResolver obj's underlying member method code with example usage: obj.iter_subconfigs(cls, id, config) without return types