[CI] Only capture a single CUDA graph size in CI by default (#25951)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/conftest.py

@@ -731,6 +731,9 @@ class VllmRunner:
         init_ctx = (nullcontext() if default_torch_num_threads is None else
                     set_default_torch_num_threads(default_torch_num_threads))
 
+        if not kwargs.get("compilation_config", None):
+            kwargs["compilation_config"] = {"cudagraph_capture_sizes": [8]}
+
         with init_ctx:
             self.llm = LLM(
                 model=model_name,