Push Message formatting into _gen_model_input (#1295)

* Revert Generate Behavior for non-Flamingo Models

* Simplify

torchchat/generate.py

@@ -797,6 +797,10 @@ def _gen_model_input(
             max_new_tokens is not None
         ), "max_new_tokens must be specified for Flamingo models"
 
+        # Wrap string prompts into a list
+        if isinstance(prompt, str):
+            prompt = [{"role": "user", "content": prompt}]
+
         image_found = False
         messages = []
         for message in prompt:
@@ -959,8 +963,9 @@ def chat(
         max_seq_length = (
             text_transformer_args.max_seq_length if text_transformer_args else 2048
         )
+
         encoded, batch = self._gen_model_input(
-            [{"role": "user", "content": generator_args.prompt}],
+            generator_args.prompt,
             generator_args.image_prompts,
             generator_args.max_new_tokens,
             max_seq_length,