[main][bugfix] Fix spec acceptance rate problem in vllm_0.15.0 (#6606)

### What this PR does / why we need it?
The speculative inference acceptance rate decreases after the vllm
version is upgraded to v0.15.0. This issue is resolved.

### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?
UT and tests case

- vLLM version: v0.15.0
- vLLM main:
d7e17aaacd

---------

Signed-off-by: lilinsiman <lilinsiman@gmail.com>

vllm_ascend/ascend_forward_context.py

@@ -19,6 +19,7 @@ from vllm_ascend.utils import (
     is_drafter_moe_model,
     is_moe_model,
     speculative_enable_dispatch_gmm_combine_decode,
+    vllm_version_is,
 )
 
 
@@ -152,6 +153,9 @@ def set_ascend_forward_context(
                 mc2_mask[num_actual_tokens:] = False
                 forward_context.mc2_mask = mc2_mask
 
+        if is_draft_model and vllm_version_is("0.15.0"):
+            forward_context.remaining_moe_layers = None
+
         try:
             yield
         finally: