Fix formatting of model subclass pipelining examples

Summary:
One of the code examples was not showing and the other was showing the whole file rather than just the relevant part.

TF2.5 Only

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

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

Maniphest Tasks: T58378

Differential Revision: https://phabricator.sourcevertex.net/D63199

Author: Alfie-Edwards

tensorflow/compiler/plugin/poplar/docs/keras_tf2.rst

@@ -210,8 +210,8 @@ different pipeline stages as follows:
 .. literalinclude:: keras_tf2_example11.py
   :language: python
   :linenos:
-  :start-at: input_layer = tf.keras.layers.Input((28, 28))
-  :end-at: model = tf.keras.Model(inputs=input_layer, outputs=x)
+  :start-at: class MyModel(tf.keras.Model):
+  :end-at:     return x
 
 .. note::
 Layers *constructed* within an `ipu.keras.PipelineStage` context will have that
@@ -253,7 +253,8 @@ to four different pipeline stages as follows:
 .. literalinclude:: keras_tf2_example12.py
   :language: python
   :linenos:
-  :start-at: strategy = ipu.ipu_strategy.IPUStrategy()
+  :start-at:   model = ExistingModel()
+  :end-at:   model.set_pipeline_stage_assignment(assignments)
 
 .. note::