add pip install setuptools in flash-attn docs (#11271)
## Summary
I followed the docs to install flash-attn with uv and got the following
issue
```uv sync
Resolved 27 packages in 12ms
× Failed to build `flash-attn==2.7.4.post1`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)
[stderr]
Traceback (most recent call last):
File "<string>", line 8, in <module>
ModuleNotFoundError: No module named 'setuptools'
hint: This usually indicates a problem with the package or the build environment.
```
installing setuptools before running uv sync as done with torch helps
fix it.
## Test Plan
I tested locally before it failed to install, after it worked
