Add requirements.txt for reproducibility (fixes #176)

[kamalahasiniburra]

Summary

Addresses Issue #176. This PR adds a requirements.txt file specifying all core dependencies needed to run the DeepLense pipeline.

What's Included

• Core dependencies with minimum version pins (PyTorch, NumPy, scikit-learn, etc.)
• • Optional dependencies clearly marked (astropy, wandb)
• • Development dependencies commented out (black, isort, flake8)
    This enables new contributors to set up their environment with a single pip install -r requirements.txt command.