CUDA extension build requirements missing from README

[anselmotalotta]

Summary

Several dependencies (e.g., flash-attn, possibly others) require:

• full CUDA toolkit with headers,
• nvcc,
• gcc <= 12 (CUDA compiler compatibility).

The README suggests only installing PyTorch with CUDA support, which is insufficient.

Result

• Users with only NVIDIA drivers (no toolkit) cannot build dependencies.
• Builds fail on distros where gcc >= 13.
• Installation breaks even before training/evaluation begins.

Requested Fixes

• Add a "System Requirements" section specifying:
  • CUDA toolkit version needed (e.g., CUDA 12.1),
  • compatible gcc version range,
  • alternatives for users without CUDA toolkit.
• Provide a Dockerfile or a prebuilt environment where all dependencies compile successfully.