docstr: Get (outer_port, node) keys of the output placeholders at the innermost level.
  
  When the final step of a model is a sub-model and expand_nested is True, we plot
  the sub-models internal outputs as the model outputs. It the sub-model itself
  contains a sub-model, we continue recursively until hitting a non-Model step.
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code: def get_innermost_outputs_keys(self, model, outer_port):
      """Get (outer_port, node) keys of the output placeholders at the innermost level.
  
      When the final step of a model is a sub-model and expand_nested is True, we plot
      the sub-models internal outputs as the model outputs. It the sub-model itself
      contains a sub-model, we continue recursively until hitting a non-Model step.
      """
      keys = []
      for output in model._internal_outputs:
          if self.expand_nested and _is_model(output.node):
              keys.extend(
                  self.get_innermost_outputs_keys(
                      output.node.step, output.port
                  )
              )
          else:
              keys.append((outer_port, output))
      return keys
  
file_path: baikal/baikal/plot.py
imports: import io
  import os
  from typing import Optional
  from baikal import Model
  from baikal._core.utils import make_name, safezip2
label_desc: Use image node_id 5 for calling the _DotTransformer obj's underlying member method code with example usage: obj.get_innermost_outputs_keys(model, outer_port) and returns: keys