[Models] add fleet model fallback

[xiaoguoguo626807]

Motivation

新增 PaddleFleet 作为模型推理后端（--model-impl paddlefleet），通过将 PaddleFleet TransformerLayer 中的 core_attention 替换为 FastDeploy Attention 内核，实现在 PaddleFleet 模型结构上复用 FastDeploy 的 KV Cache 和高性能 Attention 计算。

Modifications

• config.py: 新增 paddlefleet 到 ModelImpl 类型定义
• engine/args_utils.py: 支持 --model-impl paddlefleet CLI 参数，并补充校验逻辑
• model_executor/models/paddleformers/base_fleet.py: 新增 PaddleFleetModelBase 基类、FastDeployAttention 层及 patch_paddlefleet_core_attention 替换函数
• model_executor/models/paddleformers/__init__.py: 注册 PaddleFleetForCausalLM 模型类

Usage or Command

python -m fastdeploy.entrypoints.openai.api_server \
    --model /path/to/model \
    --model-impl paddlefleet

Accuracy Tests

N/A（本 PR 新增 PaddleFleet 推理后端，尚未提供与参考实现的 logits 对齐数据）

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
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• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
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[paddle-bot]

Thanks for your contribution!