Fix deterministic seed handling, add graph utils tests, improve snaps hot prefetch concurrency, and document notebook dependencies

[ritik4ever]

PR Description

This PR adds deterministic training seed handling, improves graph utility coverage, enhances snapshot concurrency, and documents notebook dependency setup.

What changed

• Added support for --seed in train.py and ASTROML_SEED fallback
• Ensured deterministic seeding across Python, NumPy, and PyTorch
• Added coverage for astroml.graph_utils.graph_to_pyg_data edge cases
• Improved astroml/features/graph/snapshot.py to support parallel snapshot prefetching
• Added notebook dependency setup instructions to example notebooks and README.md

Closes

• Closes train.py lacks deterministic seed handling #159
• Closes Feature extractor concurrency bottleneck #160
• Closes Missing tests for graph_utils.py #184
• Closes Notebook examples require environment setup instructions #155

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