docstr: Delete the SSAfunction with function_name.
code: def delete_subgraph(self, name):
      """
      Delete the SSAfunction with function_name.
      """
      if name not in self.functions:
          logging.warning("(%s) not in NetworkEnsemble", name)
          return
      del self.functions[name]
  
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 5 for calling the NetworkEnsemble obj's underlying member method code with example usage: obj.delete_subgraph(name) without return types