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

Reviewed By: #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, #tensorflow, georgep

Maniphest Tasks: T58378

Differential Revision: https://phabricator.sourcevertex.net/D63186

Author: Alfie-Edwards

tensorflow/compiler/plugin/poplar/docs/api.rst

@@ -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: