docs: add macOS / CPU-only installation instructions

[amannsonii3]

Adds CPU-only installation instructions for macOS users as an alternative
to the existing CUDA 12.8 setup in step 2.

Changes:

• Added CPU wheel URLs (torch-2.7.1+cpu) as an alternative to cu128
• Added grep -v triton workaround since triton is Linux-only and has
  no macOS wheel on PyPI

Tested on: Apple M4, macOS Tahoe 26.2, Python 3.11

[yilunliao]

Hi @amannsonii3

I have a question -- do you encounter any issue when using the commands to set up the environment on CPU-only machines?

If no, I lean toward keeping the same for simplicity.