PR #17979: [keras/testing_infra] Standardise docstring usage of "Default to"

Imported from GitHub PR #17979

This is one of many PRs. Discussion + request to split into multiple PRs @ #17748
Copybara import of the project:

--
5c24834 by Samuel Marks <[email protected]>:

[keras/testing_infra/test_combinations.py,keras/testing_infra/test_utils.py] Standardise docstring usage of "Default to"

Merging this change closes #17979

FUTURE_COPYBARA_INTEGRATE_REVIEW=#17979 from SamuelMarks:keras.testing_infra-defaults-to 5c24834
PiperOrigin-RevId: 530509950

Author: SamuelMarks

keras/testing_infra/test_combinations.py

@@ -112,7 +112,7 @@ def test_foo(self):
         test or class.
       exclude_formats: A collection of Keras saved model formats to not run.
         (May also be a single format not wrapped in a collection).
-        Defaults to None.
+        Defaults to `None`.
 
     Returns:
       Returns a decorator that will run the decorated test method multiple
@@ -258,7 +258,7 @@ def test_foo(self):
         test or class.
       exclude_models: A collection of Keras model types to not run.
         (May also be a single model type not wrapped in a collection).
-        Defaults to None.
+        Defaults to `None`.
 
     Returns:
       Returns a decorator that will run the decorated test method multiple
@@ -497,12 +497,13 @@ def keras_mode_combinations(mode=None, run_eagerly=None):
 
     Args:
       mode: List of modes to run the tests. The valid options are 'graph' and
-        'eager'. Default to ['graph', 'eager'] if not specified. If a empty list
-        is provide, then the test will run under the context based on tf's
-        version, eg graph for v1 and eager for v2.
+        'eager'. If None, uses ['graph', 'eager']. If an empty
+        list is provided, then the test will run under the context based on
+        tensorflow's version, e.g., graph for v1 and eager for v2. Defaults to
+        `None`.
       run_eagerly: List of `run_eagerly` value to be run with the tests.
-        Default to [True, False] if not specified. Note that for `graph` mode,
-        run_eagerly value will only be False.
+        When None, uses [True, False]. Note that for `graph` mode,
+        run_eagerly value will only be False. Defaults to `None`.
 
     Returns:
       A list contains all the combinations to be used to generate test cases.

keras/testing_infra/test_utils.py

@@ -880,10 +880,11 @@ def get_multi_io_model(
       shared_input_branch: An optional sequence of layers to apply to a single
         input, before applying both branches to that intermediate result. If
         set, the model will take only one input instead of two. Defaults to
-        None.
+        `None`.
       shared_output_branch: An optional sequence of layers to merge the
         intermediate results produced by branch a and branch b. If set,
-        the model will produce only one output instead of two. Defaults to None.
+        the model will produce only one output instead of two.
+        Defaults to `None`.
 
     Returns:
       A multi-io model of the type specified by `get_model_type`, specified