Update anchor_generator.py

[djkmarchetto]

Enhance compatibility with SymbolicTensor using ops instead of math

Fixes

TypeError: Exception encountered when calling RetinaNetLabelEncoder.call().

must be real number, not SymbolicTensor

Arguments received by RetinaNetLabelEncoder.call():
• images=tf.Tensor(shape=(64, None, None, 3), dtype=float32)
• box_labels={'classes': 'tf.Tensor(shape=(64, 100), dtype=float32)', 'boxes': 'tf.Tensor(shape=(64, 100, 4), dtype=float32)'}

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?
• If this adds a new model, can you run a few training steps on TPU in Colab to ensure that no XLA incompatible OP are used?

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@sampathweb

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