Add macOS compatibility: exclude GPU dependencies on darwin

[yayashuxue]

Summary

This PR adds macOS compatibility to rLLM by conditionally excluding GPU-specific dependencies that are not compatible with macOS.

Changes

• Added sys_platform != 'darwin' conditions to flash-attn, deepspeed, and vllm dependencies
• Added optional [gpu] dependency group for explicit GPU installation on compatible systems
• Updated README.md with macOS installation instructions

Benefits

• ✅ Enables rLLM installation on macOS without GPU dependencies
• ✅ Maintains full functionality for Linux/CUDA users
• ✅ Provides backward compatibility
• ✅ Minimal changes, easy to maintain

Testing

• Tested installation on macOS (darwin) - ✅ Works
• Verified Linux compatibility is preserved - ✅ No breaking changes
• GPU features can still be installed explicitly on compatible systems - ✅ Optional group works

Related Issues

Fixes installation issues on macOS systems where GPU dependencies are not available or compatible.

[jeffreysijuntan]

This is great. We recently updated our README so there is a merge conflict. Can you fix the conflict and then I will merge it in?

[yayashuxue]

Fixed! Ready to merge.