Adding ModelExtensions to ipu.keras.extensions namespace

Summary:
Adding ModelExtensions to the ipu.keras.extensions namespace. SequentialExtensions and FuntionalExtensions are both in there.

TF2.5 Only

Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, georgep

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

Maniphest Tasks: T58378

Differential Revision: https://phabricator.sourcevertex.net/D63220

Author: Alfie-Edwards

tensorflow/python/ipu/keras/extensions/__init__.py

@@ -23,4 +23,6 @@
 from tensorflow.python.ipu.keras.extensions.functional_extensions import FunctionalExtension
 from tensorflow.python.ipu.keras.extensions.sequential_extensions import SequentialLayerPipelineStageAssignment
 from tensorflow.python.ipu.keras.extensions.sequential_extensions import SequentialExtension
+from tensorflow.python.ipu.keras.extensions.model_extensions import ModelLayerPipelineStageAssignment
+from tensorflow.python.ipu.keras.extensions.model_extensions import ModelExtension
 # pylint: enable=unused-import