[Main2Main][Deps][Misc] Upgrade vLLM to v0.15.0 (#6470)

### What this PR does / why we need it?
This PR upgrades the vLLM dependency from `v0.14.1` to `v0.15.0`. This
involves:
- Updating the `VLLM_TAG` in all `Dockerfile`.
- Updating the vLLM version in `docs/source/conf.py`.
- Removing conditional code paths specific to `v0.14.1` across the
codebase, which simplifies maintenance.
- Fix `TypeError: MMEncoderAttention.__init__() got an unexpected
keyword argument 'multimodal_config'` due to
https://github.com/vllm-project/vllm/pull/31972.
- Fix `_shared_experts: 'NoneType' object is not callable` due to
https://github.com/vllm-project/vllm/pull/32082 by
https://github.com/vllm-project/vllm-ascend/pull/6335.
- Fix `ReshapeAndCacheOperation setup failed!` due to
https://github.com/vllm-project/vllm/pull/25954 by overriding attention
metadata slots.

This upgrade is necessary to keep the project aligned with the latest
features, bug fixes, and API changes in the vLLM project.

### Does this PR introduce _any_ user-facing change?
No, this is an internal dependency update and does not introduce any
user-facing changes.

### How was this patch tested?
CI is expected to pass with these changes, ensuring that all existing
tests are successful with the new vLLM version.

- vLLM version: v0.14.1
- vLLM main:
dc917cceb8


co-authored-by: shen-shanshan <467638484@qq.com>

---------

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

vllm_ascend/ops/fused_moe/fused_moe.py

@@ -512,6 +512,14 @@ class AscendSharedFusedMoE(SharedFusedMoE, AscendFusedMoE):
         hidden_states: torch.Tensor,
         router_logits: torch.Tensor,
     ) -> tuple[torch.Tensor, torch.Tensor]:
+        if self._shared_experts is None:
+            fused_out = AscendFusedMoE.forward(
+                self,
+                hidden_states=hidden_states,
+                router_logits=router_logits,
+            )
+            shared_out = None
+            return shared_out, fused_out
         shared_out, fused_out = AscendFusedMoE.forward(
             self,
             hidden_states=hidden_states,
@@ -571,6 +579,9 @@ class AscendSharedFusedMoE(SharedFusedMoE, AscendFusedMoE):
         )
         routed_out = fused_moe_results.routed_out
 
+        if self._shared_experts is None:
+            return routed_out
+
         if self.multistream_overlap_gate:
             fc3_context = get_flash_common3_context()
             assert fc3_context is not None