[CAR] Let custom allreduce support VMM based allocation (#27593)

Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>

test/registered/spec/eagle/test_deepseek_v3_fp4_mtp_small.py

@@ -48,11 +48,15 @@ class TestDeepseekV3FP4MTP(CustomTestCase):
             "--model-loader-extra-config",
             '{"enable_multithread_load": true,"num_threads": 64}',
         ]
+        env = {
+            "PYTORCH_CUDA_ALLOC_CONF": "expandable_segments:True",
+        }
         cls.process = popen_launch_server(
             cls.model,
             cls.base_url,
             timeout=SERVER_LAUNCH_TIMEOUT,
             other_args=other_args,
+            env=env,
         )
 
     @classmethod