feat(flax): allow encoder_outputs in generate (#15554)

* feat(flax): allow encoder_outputs in generate

* doc(flax): encoder_outputs in generate

* fix: style

* fix: style

src/transformers/generation_flax_utils.py

@@ -217,7 +217,9 @@ def generate(
             params (`Dict[str, jnp.ndarray]`, *optional*):
                 Optionally the model parameters can be passed. Can be useful for parallelized generation.
             model_kwargs:
-                Additional model specific kwargs will be forwarded to the `forward` function of the model.
+                Additional model specific kwargs will be forwarded to the `forward` function of the model. If the model
+                is an encoder-decoder model, encoder specific kwargs should not be prefixed and decoder specific kwargs
+                should be prefixed with *decoder_*. Also accepts `encoder_outputs` to skip encoder part.
 
         Return:
             [`~file_utils.ModelOutput`].
@@ -251,7 +253,8 @@ def generate(
 
         if self.config.is_encoder_decoder:
             # add encoder_outputs to model_kwargs
-            model_kwargs = self._prepare_encoder_decoder_kwargs_for_generation(input_ids, params, model_kwargs)
+            if model_kwargs.get("encoder_outputs") is None:
+                model_kwargs = self._prepare_encoder_decoder_kwargs_for_generation(input_ids, params, model_kwargs)
             # prepare decoder_input_ids for generation
             input_ids = jnp.ones((input_ids.shape[0], 1), dtype="i4") * decoder_start_token_id