This fix goes beyond the requirements of the issue and adds support for handling Keras models with dictionary-based inputs, particularly when exporting to the TFSavedModel format for both the TensorFlow and JAX backends. Previously, models with dictionary inputs would fail during export with ValueErrors related to input structure mismatches.

Key changes:

• Added proper handling of model._input_names for dictionary-based inputs in Functional and Model classes
• Enhanced input signature generation to properly handle dictionary input structures
• Added test coverage for dictionary input model export
• Added debug logging throughout the input signature handling flow

This PR aims to fix #20835 where models with dictionary inputs would fail to export properly to SavedModel format.

Example of fixed functionality:

inputs = {
  "foo": layers.Input(shape=()),
  "bar": layers.Input(shape=()),
}
outputs = layers.Add()([inputs["foo"], inputs["bar"]])
model = models.Model(inputs, outputs)
ref_input = {"foo": tf.constant([1.0]), "bar": tf.constant([2.0])}
ref_output = model(ref_input)
model.export('/test', format="tf_saved_model")
revived_model = tf.saved_model.load(temp_filepath)
revived_output = revived_model.serve(ref_input)