[diffusion] fix: fix CLIP text encoder attention mask not used (#14364)

Co-authored-by: niehen6174 <niehen.6174@gmail.com>
Co-authored-by: Mick <mickjagger19@icloud.com>

python/sglang/multimodal_gen/configs/models/encoders/clip.py

@@ -9,6 +9,7 @@ from sglang.multimodal_gen.configs.models.encoders.base import (
     TextEncoderArchConfig,
     TextEncoderConfig,
 )
+from sglang.multimodal_gen.runtime.platforms import AttentionBackendEnum
 
 
 def _is_transformer_layer(n: str, m) -> bool:
@@ -38,6 +39,11 @@ class CLIPTextArchConfig(TextEncoderArchConfig):
     bos_token_id: int = 49406
     eos_token_id: int = 49407
     text_len: int = 77
+    _supported_attention_backends: set[AttentionBackendEnum] = field(
+        default_factory=lambda: {
+            AttentionBackendEnum.TORCH_SDPA,  # Force TORCH_SDPA to support attention_mask
+        }
+    )
     stacked_params_mapping: list[tuple[str, str, str]] = field(
         default_factory=lambda: [
             # (param_name, shard_name, shard_id)