[Test][BugFix] Fix dispatch_gmm_combine_decode test stability (#7097)

### What this PR does / why we need it?
This patch fix the nightly failure
1. Each case uses a copy of the global kwargs instead of a reference to
prevent parameter pollution between use cases.
2. Add weight initialization in the scenario of `eplb` + `w8a8_dynamic`

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?
```python
pytest -sv tests/e2e/nightly/single_node/ops/multicard_ops_a3/test_dispatch_gmm_combine_decode.py
```

```shell
===================================================================== 3 passed, 4 warnings in 194.86s (0:03:14) ======================================================================
```
- vLLM version: v0.16.0
- vLLM main:
4034c3d32e

Signed-off-by: wangli <wangli858794774@gmail.com>

tests/e2e/nightly/single_node/ops/multicard_ops_a3/test_dispatch_gmm_combine_decode.py

@@ -320,6 +320,9 @@ class FusionOp(DecodeMoeOps):
                 self.gmm2_weight_scale_fp32 = [
                     weight.clone() for weight in gmm2_weight_scale.unbind(dim=0)
                 ]
+            else:
+                self.gmm1_weight_scale_fp32 = [torch.ones(1).npu().to(gmm1_weight.dtype)]
+                self.gmm2_weight_scale_fp32 = [torch.ones(1).npu().to(gmm2_weight.dtype)]
         else:
             self.gmm1_weight = [gmm1_weight.clone()]
             self.gmm2_weight = [gmm2_weight.clone()]
@@ -524,7 +527,7 @@ def run_once(local_rank_id,
 
 @torch.inference_mode()
 def test_dispatch_gmm_combine_decode_base():
-    custom_kwargs = BASE_KWARGS
+    custom_kwargs = BASE_KWARGS.copy()
     custom_kwargs["batch_size"] = 32
     custom_kwargs["ep_world_size"] = 8
     custom_kwargs["moe_expert_num"] = 32
@@ -539,7 +542,7 @@ def test_dispatch_gmm_combine_decode_base():
 
 @torch.inference_mode()
 def test_dispatch_gmm_combine_decode_with_mc2_mask():
-    custom_kwargs = BASE_KWARGS
+    custom_kwargs = BASE_KWARGS.copy()
     custom_kwargs["with_mc2_mask"] = True
     ep_world_size = custom_kwargs["ep_world_size"]
     custom_args = tuple(custom_kwargs.values())
@@ -548,7 +551,7 @@ def test_dispatch_gmm_combine_decode_with_mc2_mask():
 
 @torch.inference_mode()
 def test_dispatch_gmm_combine_decode_dynamic_eplb():
-    custom_kwargs = BASE_KWARGS
+    custom_kwargs = BASE_KWARGS.copy()
     custom_kwargs["dynamic_eplb"] = True
     ep_world_size = custom_kwargs["ep_world_size"]
     custom_args = tuple(custom_kwargs.values())