BuildingABrain.ipynb - Improved notebook portability and fixed Keras model issues

[johnwhitegm]

Summary

This PR improves the notebook’s portability, reliability, and compatibility across environments (Google Colab, SageMaker/Kaggle, and GitHub preview) while also fixing functional issues in the model definition.

Changes Made

Fix YouTube embed (Colab Error 153)

• Replaced YouTubeVideo() with a custom iframe embed
• Added: html referrerpolicy="strict-origin-when-cross-origin"
• Ensures video renders correctly in Colab

Fix GitHub notebook preview issues

• GitHub preview was stripping or corrupting HTML inside Python strings (e.g., <iframe>)
• Resolved by dynamically constructing HTML tags (e.g., "<" + "iframe")
• Ensures code displays correctly in GitHub and HTML still renders correctly in execution environments

Fix image rendering issue

• Replaced Wikimedia thumbnail URL (/thumb/...) with direct image URL

Fix model initialization error (Keras ValueError)

• Issue: ValueError: expected a positive integer for units
• Root cause: number_of_classes was a NumPy scalar (numpy.uint8)
• Fix: number_of_classes = int(train_labels.max() + 1)
• Ensures compatibility with Keras validation logic

Resolve Keras UserWarning (best practice)

• Updated model definition to use explicit input layer: tf.keras.Input(shape=(28, 28))
• Removed input_shape argument from Flatten
• Aligns with modern Keras recommendations for Sequential models

Why This Matters

These changes ensure the notebook:

• Works consistently across Colab, SageMaker, and Kaggle
• Displays correctly in GitHub preview
• Avoids runtime errors and warnings in Keras
• Follows current best practices for model definition

Testing

• Verified in Google Colab (video + images render correctly)
• Verified model builds successfully without errors
• Verified GitHub preview displays code properly without corruption

Additional Notes

• Some behaviors (e.g., iframe rendering) are environment-specific due to:
  • Colab iframe security policies
  • GitHub HTML sanitization
• Workarounds were chosen to maintain cross-platform compatibility