tf.keras.losses.SparseCategoricalCrossentropy has logical/Doc bug

[shinjh0849]

Describe the problem.

API documentation of tf.keras.losses.SparseCategoricalCrossentropy mentions that one of the parameters can be None, but the implementation does not check None, it checks 'none' which is a string.

Describe the current behavior.

Throws a value error when running my test cases, which I think it shouldn't.
(redirected here from tensorflow/tensorflow#89246).

Describe the expected behavior.

It shouldn't throw a value error for using None instead of 'none' as per the API documentation.

Contributing.

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• Briefly describe your candidate solution(if contributing):

Standalone code to reproduce the issue.

def test_reduction_none(self):
        # Test with reduction set to None
        y_true = np.array([0, 1, 2])
        y_pred = np.array([[0.9, 0.05, 0.05],
                           [0.05, 0.9, 0.05],
                           [0.05, 0.05, 0.9]])
        loss_fn = tf.keras.losses.SparseCategoricalCrossentropy(reduction=None)
        loss = loss_fn(y_true, y_pred).numpy()
        expected_loss = -np.log([0.9, 0.9, 0.9])
        np.testing.assert_almost_equal(loss, expected_loss, decimal=5)

Provide a reproducible test case that is the bare minimum necessary to generate
the problem. If possible, please share a link to Colab/Jupyter/any notebook.

Source code / logs.

Traceback (most recent call last):
  File "/home/user/projects/api_guided_testgen/out/bug_detect_gpt4o/exec/basic_rag_apidoc/tf/tf.keras.losses.SparseCategoricalCrossentropy.py", line 49, in test_reduction_none
    loss_fn = tf.keras.losses.SparseCategoricalCrossentropy(reduction=None)
  File "/home/user/anaconda3/lib/python3.8/site-packages/keras/losses.py", line 1026, in __init__
    super().__init__(
  File "/home/user/anaconda3/lib/python3.8/site-packages/keras/losses.py", line 262, in __init__
    super().__init__(reduction=reduction, name=name)
  File "/home/user/anaconda3/lib/python3.8/site-packages/keras/losses.py", line 93, in __init__
    losses_utils.ReductionV2.validate(reduction)
  File "/home/user/anaconda3/lib/python3.8/site-packages/keras/utils/losses_utils.py", line 88, in validate
    raise ValueError(
ValueError: Invalid Reduction Key: None. Expected keys are "('auto', 'none', 'sum', 'sum_over_batch_size')"

Include any logs or source code that would be helpful to diagnose the problem. If including tracebacks, please include the full traceback. Large logs and files should be attached. Try to provide a reproducible test case that is the bare minimum necessary to generate the problem.