[V1] Make V1 engine backward compatible (#637)

### What this PR does / why we need it?
Enforce eager mode in the V1 engine ahead of the upcoming CANN and
torch_npu releases.

### Does this PR introduce _any_ user-facing change?
After this change, users will no longer need to manually set
enforce_eager=True.

### How was this patch tested?
Test it with regular offline inference examples.

Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>

tests/singlecard/test_offline_inference.py

@@ -52,7 +52,7 @@ def test_models(model: str, dtype: str, max_tokens: int) -> None:
     with VllmRunner(model,
                     max_model_len=8192,
                     dtype=dtype,
-                    enforce_eager=True,
+                    enforce_eager=False,
                     gpu_memory_utilization=0.7) as vllm_model:
         vllm_model.generate_greedy(example_prompts, max_tokens)