[Bug] Fix workspace manager `_current_workspaces` size (#38853)

Signed-off-by: yewentao256 <zhyanwentao@126.com>

vllm/v1/worker/workspace.py

@@ -31,7 +31,7 @@ _manager: "WorkspaceManager | None" = None
 class WorkspaceManager:
     """Manager for workspace allocation.
 
-    Manages workspace buffers for DBO (Dual Batch Overlap) execution.
+    Manages one workspace buffer per active ubatch slot.
     Can be locked to prevent further growth during execution.
     """
 
@@ -39,7 +39,9 @@ class WorkspaceManager:
         self._device = device
         # Cache num ubatches at init based on configuration (default to 1)
         self._num_ubatches = num_ubatches if num_ubatches is not None else 1
-        self._current_workspaces: list[torch.Tensor | None] = [None, None]
+        self._current_workspaces: list[torch.Tensor | None] = [
+            None
+        ] * self._num_ubatches
         self._locked: bool = False
 
     @staticmethod
@@ -224,7 +226,7 @@ def init_workspace_manager(
 
     Args:
         device: The device to allocate workspace on.
-        num_ubatches: Number of micro-batches. Defaults to 1.
+        num_ubatches: Number of workspace ubatch slots. Defaults to 1.
     """
     global _manager
     if _manager is not None: