docstr: Iterates over graph and returns input format (`NCHW` or `NHWC`)
  if input is of type Image, otherwise `None`
code: def get_image_format(self):
      """
      Iterates over graph and returns input format (`NCHW` or `NHWC`)
      if input is of type Image, otherwise `None`
      """
      for fn_key in list(self.functions.keys()):
          graph = self.functions[fn_key].graph
  
          for name in graph:
              node = graph[name]
              if (
                  node.attr.get("data_format", None) == "NHWC"
                  or node.attr.get("data_format")
                  == "NHWC_format_inserted"
              ):
                  return "NHWC"
              elif node.attr.get("data_format", None) == "NCHW":
                  return "NCHW"
      return None
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  import copy
  import logging
  from coremltools.converters.mil.mil import types
  from .basic_graph_ops import check_connections, const_determined_nodes
  from .dot_visitor import DotVisitor
  from .naming_utils import escape_fn_name
label_desc: Use image node_id 15 for calling the NetworkEnsemble obj's underlying member method code with example usage: obj.get_image_format() and returns: None, str, str