Fix Keras serialization for losses and NeuralDecisionTree

[jrosenfeld13]

• Add @register_keras_serializable and get_config() to SpearmanCorrelation
• Add @register_keras_serializable and get_config() to CombinedLoss
• Add @register_keras_serializable and get_config() to NeuralDecisionTree
• Replace lambda initializer with Constant() in NeuralDecisionTree

Fixes pickle/model.save() for all custom Keras classes.

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Note

Improves Keras save/load compatibility for custom components.

• Adds @register_keras_serializable and get_config() to SpearmanCorrelation and CombinedLoss (exposes regularization_strength, mse_weight, spearman_weight, spearman_regularization)
• Adds @register_keras_serializable, **kwargs passthrough, stored init params, and get_config() to NeuralDecisionTree
• Replaces lambda initializer with initializers.Constant for temperature and feature mask weights

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