message: Adding Model subclasses to api docs
  Summary:
  Adding ModelExtensions to the API docs in the same section as FunctionalExtensions and SequentialExtensions.
  TF2.5 Only
  Reviewers: #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, jamiep, jackh, #tensorflow, georgep
  Maniphest Tasks: T58378
diff: @@ -156,8 +156,8 @@ Keras
  ^^^^^
  .. automodule:: tensorflow.python.ipu.keras.extensions
  - :members: FunctionalExtension, PipelineStage, FunctionalLayerPipelineStageAssignment, SequentialExtension, SequentialLayerPipelineStageAssignment
  - :imported-members: FunctionalExtension, PipelineStage, FunctionalLayerPipelineStageAssignment, SequentialExtension, SequentialLayerPipelineStageAssignment
  + :members: FunctionalExtension, PipelineStage, FunctionalLayerPipelineStageAssignment, SequentialExtension, SequentialLayerPipelineStageAssignment, ModelExtension, ModelLayerPipelineStageAssignment
  + :imported-members: FunctionalExtension, PipelineStage, FunctionalLayerPipelineStageAssignment, SequentialExtension, SequentialLayerPipelineStageAssignment, ModelExtension, ModelLayerPipelineStageAssignment
  .. _keras-layers-api:
  