[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/attention/sfa_v1.py

@@ -9,6 +9,7 @@ from vllm.config import VllmConfig, get_current_vllm_config
 from vllm.distributed import get_tensor_model_parallel_world_size, get_tp_group
 from vllm.forward_context import get_forward_context
 from vllm.logger import logger
+from vllm.model_executor.layers.attention.mla_attention import MLACommonMetadataBuilder
 from vllm.model_executor.layers.linear import UnquantizedLinearMethod
 from vllm.triton_utils import HAS_TRITON
 from vllm.v1.attention.backend import AttentionBackend, AttentionCGSupport, MLAAttentionImpl  # type: ignore
@@ -45,17 +46,11 @@ from vllm_ascend.utils import (
     enable_dsa_cp,
     enable_dsa_cp_with_layer_shard,
     maybe_trans_nz,
-    vllm_version_is,
 )
 from vllm_ascend.worker.npu_input_batch import NPUInputBatch
 
 if TYPE_CHECKING:
     from vllm.v1.core.sched.output import SchedulerOutput
-if vllm_version_is("0.14.1"):
-    from vllm.v1.attention.backends.mla.common import MLACommonMetadataBuilder  # type: ignore
-else:
-    from vllm.model_executor.layers.attention.mla_attention import MLACommonMetadataBuilder
-# isort: on
 
 # token count limits within bmm_transpose operator
 BMM_TRANS_MAX_SUPPORTED_TOKENS = 1024