Refactor Keras engine by splitting it into short, specialized files.

The purpose of this change is to make the codebase more maintainable and readable.

Before:
engine/topology.py
models.py

After:
engine/base_layer.py
engine/input_layer.py
engine/network.py
engine/sequential.py
engine/saving.py

This is a large change but it only moves code around with no change in logic or API.

New files are all under 1000 lines of logic (network.py is 1500 lines, but under 1000 if you remove imports and docstrings), and often under 500.

PiperOrigin-RevId: 186847895