Add CPU/GPU compatibility and automatic device selection

[JimmyOhn]

Resolves memory issues and improves accessibility on CPU-only systems.
- Add device parameter to SyncNetInstance with CPU as default
- Implement automatic CUDA detection and fallback to CPU
- Update all scripts (demo_syncnet.py, demo_feature.py, run_syncnet.py, run_pipeline.py)
  to use automatic device selection
- Add device_utils.py for device management utilities
- Modify S3FD face detector to support CPU/GPU selection
- Ensure backward compatibility with existing CUDA setups
- Enable SyncNet to run on systems without NVIDIA GPU