[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>

.github/workflows/_e2e_nightly_multi_node.yaml

@@ -32,7 +32,7 @@ on:
         description: how many pods will be pulled up via lws.yaml, indicates number of nodes we need
       vllm_version:
         required: false
-        default: "v0.14.1"
+        default: "v0.15.0"
         type: string
         description: vllm version to use
       vllm_ascend_remote_url:
@@ -78,7 +78,7 @@ jobs:
         - name: Decode kubeconfig from secrets
           run: |
             # Decode and save kubeconfig
-            echo "${{ secrets.KUBECONFIG_B64 }}" | base64 -d > $KUBECONFIG
+            echo "${{ secrets.KUBECONFIG_B64 }}" | base64 -d > "$KUBECONFIG"
 
         - name: Checkout code
           uses: actions/checkout@v6
@@ -133,7 +133,7 @@ jobs:
             image="${{ inputs.image }}"
             config_file_path="${{ inputs.config_file_path }}"
             fail_tag=FAIL_TAG_"${{ inputs.config_file_path }}"
-            echo "FAIL_TAG=${fail_tag}" >> $GITHUB_ENV
+            echo "FAIL_TAG=${fail_tag}" >> "$GITHUB_ENV"
 
             required_params=("size" "replicas" "image" "config_file_path")
             for param in "${required_params[@]}"; do
@@ -264,5 +264,5 @@ jobs:
         - name: Post process
           if: always()
           run: |
-            kubectl get pods -n $NAMESPACE --ignore-not-found=true
+            kubectl get pods -n "$NAMESPACE" --ignore-not-found=true
             kubectl delete -f ./lws.yaml --ignore-not-found=true || true