[Kernel] Build flash-attn from source (#8245)

.gitignore

@@ -1,6 +1,9 @@
 # vllm commit id, generated by setup.py
 vllm/commit_id.py
 
+# vllm-flash-attn built from source
+vllm/vllm_flash_attn/
+
 # Byte-compiled / optimized / DLL files
 __pycache__/
 *.py[cod]
@@ -12,6 +15,8 @@ __pycache__/
 # Distribution / packaging
 .Python
 build/
+cmake-build-*/
+CMakeUserPresets.json
 develop-eggs/
 dist/
 downloads/