Fix multi_layer_eagle_worker_v2 draft extend selection, add chain style multi layer mtp test (#22340)

Co-authored-by: 0xNullPath <luyan@nvidia.com>

python/sglang/srt/speculative/multi_layer_eagle_worker_v2.py

@@ -27,6 +27,7 @@ from sglang.srt.managers.tp_worker import TpModelWorker
 from sglang.srt.model_executor.forward_batch_info import CaptureHiddenMode, ForwardBatch
 from sglang.srt.server_args import ServerArgs
 from sglang.srt.speculative.base_spec_worker import BaseDraftWorker, BaseSpecWorker
+from sglang.srt.speculative.draft_utils import DraftBackendFactory
 from sglang.srt.speculative.eagle_info import EagleDraftInput, EagleVerifyInput
 from sglang.srt.speculative.eagle_info_v2 import fill_new_verified_id
 from sglang.srt.speculative.eagle_utils import TreeMaskMode, build_tree_kernel_efficient
@@ -183,16 +184,14 @@ class MultiLayerEagleDraftWorker(BaseDraftWorker):
         # Create attn backends
         self.draft_extend_attn_backend_list = []
         for step in range(self.speculative_num_steps):
-            from sglang.srt.layers.attention.flashattention_backend import (
-                FlashAttentionBackend,
+            draft_backend_factory = DraftBackendFactory(
+                self.server_args,
+                self.draft_runner_list[step],
+                self.topk,
+                self.speculative_num_steps,
             )
-
             self.draft_extend_attn_backend_list.append(
-                FlashAttentionBackend(
-                    model_runner=self.draft_runner_list[step],
-                    skip_prefill=False,
-                    speculative_step_id=step,
-                )
+                draft_backend_factory.create_draft_extend_backend()
             )
             self.draft_runner_list[step].attn_backend = (
                 self.draft_extend_attn_backend_list[-1]