[Bugfix] Fixed an accuracy problem of sp with eagle3 (#5816)

### What this PR does / why we need it?
Fixed an accuracy problem when using eagle3 with sp.

The problem is described in
https://github.com/vllm-project/vllm-ascend/issues/5825.

It also adds a much more precise way to determine whether drafter should
use `sp` or not.

Also, it changes the `eager` of drafter to be a real `eager` in frontend
to avoid a `fx-graph` problem.

### Does this PR introduce _any_ user-facing change?

N/A

### How was this patch tested?

For simpilicity, we test it as in
https://github.com/vllm-project/vllm-ascend/issues/5825.

And we get the same result of `eagle3` with `sp` disabled.

```text
--------------------------------------------------
total_num_output_tokens: 1000
num_drafts: 437
num_draft_tokens: 1311
num_accepted_tokens: 564
mean acceptance length: 2.29
--------------------------------------------------
acceptance at token 0: 0.62
acceptance at token 1: 0.40
acceptance at token 2: 0.27
acceptance at token 3: 0.00
acceptance at token 4: 0.00
acceptance at token 5: 0.00
```

* vLLM version: v0.13.0
* vLLM main:
2f4e6548ef

Signed-off-by: drslark <slarksblood@qq.com>

.github/workflows/_e2e_test.yaml

@@ -217,7 +217,7 @@ jobs:
           pytest -sv --durations=0 tests/e2e/multicard/2-cards/test_external_launcher.py
           pytest -sv --durations=0 tests/e2e/multicard/2-cards/test_full_graph_mode.py
           pytest -sv --durations=0 tests/e2e/multicard/2-cards/test_ilama_lora_tp2.py
-          
+          pytest -sv --durations=0 tests/e2e/multicard/2-cards/spec_decode/test_spec_decode.py
 
           # To avoid oom, we need to run the test in a single process.
           pytest -sv --durations=0 tests/e2e/multicard/2-cards/test_offline_inference_distributed.py::test_deepseek_multistream_moe_tp2