Add JSON-based save and load functionality to Micrograd models

[Yegon-David]

This PR adds support for saving and loading Micrograd models using JSON format.

• Implemented _data() to serialize Value nodes and their computation graph to a dictionary.
• Added @staticmethod data_() to reconstruct the graph from the saved dictionary.
• Supports basic operations: +, *, **, tanh, ReLU (with correct backward functions).
• Enables saving models to disk and loading them back for continued training or inference.

This makes Micrograd models portable and easier to persist across sessions.