feat : NEFTune 噪声注入 + label_smoothing 默认值修正

- Embedding.forward 训练时注入 randn 噪声，缩放系数 neftune_noise_alpha / sqrt(seq_len)
- TrainConfig.neftune_alpha 通过 config 传递（默认 0=关闭）
- TrainContextBuilder 将 config.neftune_alpha 写入 embed_tokens
- --neftune_alpha CLI 参数（典型值 5.0）
- label_smoothing 默认值 0.05 -> 0.0

astrai/config/train_config.py

@@ -128,6 +128,10 @@ class TrainConfig(BaseConfig):
         default=1000,
         metadata={"help": "Number of optimizer steps between validation runs."},
     )
+    neftune_alpha: float = field(
+        default=0.0,
+        metadata={"help": "NEFTune noise alpha (0=disabled, typical: 5.0)."},
+    )
 
     executor_kwargs: dict = field(
         default_factory=dict,