refactor(spec): split EAGLEWorker into BaseSpecWorker/BaseDraftWorker… (#1080)

* refactor(spec): split EAGLEWorker into BaseSpecWorker/BaseDraftWorker + EagleDraftWorker (P1-2)

Author: pengchengneo

python/sgl_jax/srt/speculative/base_worker.py

@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING
+
+import jax
+from jax.sharding import NamedSharding
+from jax.sharding import PartitionSpec as P
+
+if TYPE_CHECKING:
+    from sgl_jax.srt.managers.tp_worker import ModelWorker
+
+
+def replicate_to_mesh(
+    mesh: jax.sharding.Mesh, *arrs: jax.Array
+) -> tuple[jax.Array, ...] | jax.Array:
+    """Replicate arrays across a mesh under explicit sharding.
+
+    JIT outputs are typically vocab/data-sharded; spec-decode host orchestration
+    (top_k, gather, build_tree) needs replicated arrays.
+    """
+    out = jax.device_put(arrs, NamedSharding(mesh, P()))
+    return out[0] if len(out) == 1 else out
+
+
+class BaseDraftWorker(ABC):
+    """Draft model worker interface for speculative decoding.
+
+    Concrete implementations hold the draft model runner and own all
+    draft-specific logic (multi-step decode, tree building, extend).
+    Standard EAGLE uses ``EagleDraftWorker``; MTP will use
+    ``MultiLayerDraftWorker``.
+    """
+
+    @abstractmethod
+    def draft(self):
+        pass
+
+
+class BaseSpecWorker(ABC):
+    """Speculative decode orchestrator.
+
+    Owns a ``target_worker`` (the full model) and a ``draft_worker``
+    (the draft/MTP model).  Concrete subclasses implement the main
+    entry point and connect their specific draft/verify logic.
+    """
+
+    @property
+    @abstractmethod
+    def target_worker(self) -> ModelWorker:
+        pass
+
+    @property
+    @abstractmethod
+    def draft_worker(self) -> BaseDraftWorker:
+        pass