Generate: force cache with `inputs_embeds` forwarding (#24639)

src/transformers/generation/utils.py

@@ -1304,7 +1304,12 @@ class GenerationMixin:
         # 4. Define other model kwargs
         model_kwargs["output_attentions"] = generation_config.output_attentions
         model_kwargs["output_hidden_states"] = generation_config.output_hidden_states
-        model_kwargs["use_cache"] = generation_config.use_cache
+        # decoder-only models with inputs_embeds forwarding must use caching (otherwise we can't detect whether we are
+        # generating the first new token or not, and we only want to use the embeddings for the first new token)
+        if not self.config.is_encoder_decoder and model_input_name == "inputs_embeds":
+            model_kwargs["use_cache"] = True
+        else:
+            model_kwargs["use_cache"] = generation_config.use_cache
 
         accepts_attention_mask = "attention_mask" in set(inspect.signature(self.forward).parameters.keys())
         requires_attention_mask = "encoder_outputs" not in model_kwargs