fix: prevent arbitrary code execution in torch.load()

[rajpratham1]

Security fix for vulnerability where torch.load() was used without weights_only=True, allowing potential arbitrary code execution.

Changes:

• Add secure checkpoint loading with weights_only=True by default
• Implement fallback to weights_only=False for backward compatibility
• Add security warning when unsafe loading is used
• Add documentation comments in demo.py
• Add comprehensive test suite (test_security_fix.py)

Impact:

• Prevents arbitrary code execution from malicious checkpoints
• Maintains full backward compatibility
• No breaking changes

Testing:

• All tests passing (test_security_fix.py)
• Verified safe checkpoint loading
• Verified malicious checkpoint blocking
• Verified backward compatibility