Updating TF to Keras migration guide

[rcrowe-google]

I've only updated the notebook, so hopefully the .md and .py files will be updated by the CI.

[gemini-code-assist]

Summary of Changes

Hello @rcrowe-google, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on enhancing the Keras 2 to Keras 3 migration guide, specifically the Jupyter notebook version. The changes aim to provide a more robust and user-friendly experience by clarifying instructions, updating code examples with better error handling, and refining the overall content to reflect current best practices for migrating TensorFlow Keras code to the multi-backend Keras 3 framework.

Highlights

• Guide Content Refinement: The migration guide's title has been updated to explicitly mention 'TensorFlow Keras 2 code (tf.keras)', and the content has been revised for clarity and accuracy, including updated author information and last modified date.
• Code Example Robustness: Numerous code examples throughout the notebook have been enhanced with try/except blocks to prevent CI failures and provide more informative error handling, particularly for jit_compile issues, SavedModel operations, TF Autograph, TF variable tracking, and None entries in call() arguments.
• Installation and Setup: The installation instruction was updated from keras-nightly to keras, and version printing for Keras and TensorFlow was added to the setup section for better context.
• Removed Section: The section discussing issues with 'Using deeply nested inputs in Functional Models' has been removed from the guide.
• State-Building Fixes: The guidance for state-building issues has been improved, including an updated error message example and a more complete fix that demonstrates implementing a build() method for custom layers.

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[gemini-code-assist]

Code Review

This pull request provides a significant update to the Keras 2 to Keras 3 migration guide. The changes include updating package names, adding runnable code examples for reproducing errors and demonstrating fixes, and clarifying explanations. These are excellent improvements that will make the guide more practical and easier to follow. However, I've noticed that two important sections detailing breaking changes related to handling nested inputs in Functional Models and the behavior of evaluate() with multi-output models have been removed. I've recommended re-adding them to ensure the guide remains comprehensive for users migrating from Keras 2. Additionally, I've suggested some cleanups to the operator mapping tables to remove redundancies and correct a mapping.