[FEAT] Refactor spec decode to support efficient padded speculation (#3528)

### What this PR does / why we need it?
1. Refactor the file `mtp_proposer.py`, splits torchair related codes
into `mtp_torchair_proposer.py`
2. According to https://github.com/vllm-project/vllm/pull/24539,
implements padded speculative decoding as described in
https://github.com/vllm-project/vllm/issues/21984.
### Does this PR introduce _any_ user-facing change?
User can use `disable_padded_drafter_batch` to disable/enable padded
speculation, default is `False`.
offline example:
```
speculative_config={"method": "deepseek_mtp", "num_speculative_tokens": 1, "disable_padded_drafter_batch": False}
```

### How was this patch tested?

- [x] egaer with pad/unpad:
- [x] aclgraph with pad/unpad
- [x] torchair with pad/unpad

performance test of deepseek-r1 with tp16、dp1
aclgraph with pad ITL: 168ms
aclgraph with unpad ITL: 169ms
original: 178ms


- vLLM version: v0.11.0rc3
- vLLM main:
83f478bb19

---------

Signed-off-by: xuyexiong <xuyexiong@huawei.com>

tests/e2e/singlecard/spec_decode_v1/test_v1_mtp_correctness.py

@@ -1,11 +1,15 @@
 from __future__ import annotations
 
+import os
+
 import pytest
 from vllm import SamplingParams
 from vllm.config import CompilationConfig, CUDAGraphMode
 
 from tests.e2e.conftest import VllmRunner
 
+os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
+
 
 @pytest.fixture
 def sampling_config():
@@ -17,12 +21,12 @@ def model_name():
     return "wemaster/deepseek_mtp_main_random_bf16"
 
 
-def mtp_correctness(
-    sampling_config: SamplingParams,
-    model_name: str,
-    num_speculative_tokens: int,
-    graph_mode: CUDAGraphMode = CUDAGraphMode.PIECEWISE,
-):
+def mtp_correctness(sampling_config: SamplingParams,
+                    model_name: str,
+                    num_speculative_tokens: int,
+                    graph_mode: CUDAGraphMode = CUDAGraphMode.PIECEWISE,
+                    enforce_eager=False,
+                    disable_padded_drafter_batch=True):
     example_prompts = [
         "Hello, my name is",
         "The president of the United States is",
@@ -37,7 +41,7 @@ def mtp_correctness(
                     tensor_parallel_size=1,
                     gpu_memory_utilization=0.7,
                     max_model_len=256,
-                    enforce_eager=False) as ref_llm:
+                    enforce_eager=enforce_eager) as ref_llm:
         ref_outputs = ref_llm.generate(example_prompts, sampling_config)
 
     graph_mode_str = "PIECEWISE"
@@ -54,8 +58,9 @@ def mtp_correctness(
             speculative_config={
                 "method": "deepseek_mtp",
                 "num_speculative_tokens": num_speculative_tokens,
+                "disable_padded_drafter_batch": disable_padded_drafter_batch,
             },
-            enforce_eager=False,
+            enforce_eager=enforce_eager,
             max_model_len=2000,
             compilation_config=CompilationConfig(
                 cudagraph_mode=graph_mode_str),
@@ -82,6 +87,20 @@ def mtp_correctness(
     del spec_llm
 
 
+def test_mtp1_correctness_eager(
+    sampling_config: SamplingParams,
+    model_name: str,
+):
+    mtp_correctness(sampling_config, model_name, 1, enforce_eager=True)
+
+
+def test_mtp2_correctness_eager(
+    sampling_config: SamplingParams,
+    model_name: str,
+):
+    mtp_correctness(sampling_config, model_name, 2, enforce_eager=True)
+
+
 @pytest.mark.skip("TODO(cmq): Revert me when mtp aclgraph is fixed")
 def test_mtp1_correctness_piecewise_graph(
     sampling_config: SamplingParams,
@@ -110,3 +129,47 @@ def test_mtp2_correctness_full_graph(
     model_name: str,
 ):
     mtp_correctness(sampling_config, model_name, 2, CUDAGraphMode.FULL)
+
+
+def test_mtp1_correctness_eager_with_pad(
+    sampling_config: SamplingParams,
+    model_name: str,
+):
+    mtp_correctness(sampling_config,
+                    model_name,
+                    1,
+                    enforce_eager=True,
+                    disable_padded_drafter_batch=False)
+
+
+def test_mtp2_correctness_eager_with_pad(
+    sampling_config: SamplingParams,
+    model_name: str,
+):
+    mtp_correctness(sampling_config,
+                    model_name,
+                    2,
+                    enforce_eager=True,
+                    disable_padded_drafter_batch=False)
+
+
+@pytest.mark.skip("TODO(xyx): Revert me when mtp aclgraph is fixed")
+def test_mtp1_correctness_piecewise_graph_with_pad(
+    sampling_config: SamplingParams,
+    model_name: str,
+):
+    mtp_correctness(sampling_config,
+                    model_name,
+                    1,
+                    disable_padded_drafter_batch=False)
+
+
+@pytest.mark.skip("TODO(xyx): Revert me when mtp aclgraph is fixed")
+def test_mtp2_correctness_piecewise_graph_with_pad(
+    sampling_config: SamplingParams,
+    model_name: str,
+):
+    mtp_correctness(sampling_config,
+                    model_name,
+                    2,
+                    disable_padded_drafter_batch=False)