[diffusion] fix: fix multi-prompt generation and support multiple prompts in cli (#19960)

Signed-off-by: Lancer <maruixiang6688@gmail.com>

python/sglang/multimodal_gen/configs/sample/sampling_params.py

@@ -624,8 +624,9 @@ class SamplingParams:
         parser.add_argument(
             "--prompt",
             type=str,
+            nargs="+",
             default=SamplingParams.prompt,
-            help="Text prompt for generation",
+            help="Text prompt(s) for generation. Use space-separated values for multiple prompts, e.g., --prompt 'prompt 1' 'prompt 2'",
         )
         parser.add_argument(
             "--negative-prompt",

python/sglang/multimodal_gen/runtime/entrypoints/diffusion_generator.py

@@ -166,12 +166,19 @@ class DiffGenerator:
         """
         # 1. prepare requests
         prompts = self._resolve_prompts(sampling_params_kwargs.get("prompt"))
+        user_output_file_name = sampling_params_kwargs.get("output_file_name")
+
+        if len(prompts) > 1 and user_output_file_name is not None:
+            raise ValueError(
+                "Cannot use multiple prompts with a fixed output_file_name. "
+                "Either remove --output-file-name or use a single prompt."
+            )
+
         sampling_params_orig = SamplingParams.from_user_sampling_params_args(
             self.server_args.model_path,
             server_args=self.server_args,
             **sampling_params_kwargs,
         )
-        user_output_file_name = sampling_params_kwargs.get("output_file_name")
 
         requests: list[Req] = []
         for p in prompts: