Add vllm as a submodule

.gitmodules

@@ -0,0 +1,2 @@
+[submodule "vllm"]
+	update = checkout

pyproject.toml

@@ -38,3 +38,6 @@ build-backend = "pdm.backend"
 
 [tool.pdm]
 distribution = true
+
+[tool.pdm.dependencies]
+vllm = {path = "vllm", editable = true}