feat: Add Mistral Medium 3.5 (#3633)

* fix: clarify incompat

* fix: transformers api change upstream

* fix: add pre prop

* feat: add examples

* chore: cleanup

* chore: update readme

docs/attention.qmd

@@ -54,8 +54,10 @@ python setup.py install
 
 Requirements: Hopper or Blackwell GPUs
 
+FA4 is still a pre-release on PyPI, so `--pre` is required:
+
 ```bash
-pip install flash-attn-4
+pip install --pre flash-attn-4
 ```
 
 Or from source: