Allow `bos_token_id is None` during the generation with `inputs_embeds` (#29772)

* update

* add ut

* update

src/transformers/generation/utils.py

@@ -436,9 +436,6 @@ class GenerationMixin:
             shape = encoder_outputs.last_hidden_state.size()[:-1]
             return torch.ones(shape, dtype=torch.long, device=self.device) * -100
 
-        if bos_token_id is None:
-            raise ValueError("`bos_token_id` has to be defined when no `input_ids` are provided.")
-
         # If there is some tensor in `model_kwargs`, we can infer the batch size from it. This is helpful with
         # soft-prompting or in multimodal implementations built on top of decoder-only language models.
         batch_size = 1
@@ -449,6 +446,10 @@ class GenerationMixin:
 
         if "inputs_embeds" in model_kwargs:
             return torch.ones((batch_size, 0), dtype=torch.long, device=self.device)
+
+        if bos_token_id is None:
+            raise ValueError("`bos_token_id` has to be defined when no `input_ids` are provided.")
+
         return torch.ones((batch_size, 1), dtype=torch.long, device=self.device) * bos_token_id
 
     def _prepare_attention_mask_for_generation(

tests/generation/test_utils.py

@@ -1467,6 +1467,17 @@ class GenerationTesterMixin:
                         past_kv[i][1].shape, (batch_size, num_attention_heads, seq_length, per_head_embed_dim)
                     )
 
+    def test_generate_from_inputs_embeds_with_bos_token_id_is_none(self):
+        article = "Today a dragon flew over Paris."
+        model = AutoModelForCausalLM.from_pretrained("hf-internal-testing/tiny-random-gpt2").to(torch_device)
+        tokenizer = AutoTokenizer.from_pretrained("hf-internal-testing/tiny-random-gpt2")
+        input_ids = tokenizer(article, return_tensors="pt").input_ids.to(torch_device)
+        inputs_embeds = model.get_input_embeddings()(input_ids)
+
+        model.generate(inputs_embeds=inputs_embeds, max_length=20, bos_token_id=None)
+        with self.assertRaises(ValueError):
+            model.generate(max_length=20, bos_token_id=None)
+
     def test_generate_from_inputs_embeds_decoder_only(self):
         # When supported, tests that the decoder model can generate from `inputs_embeds` instead of `input_ids`
         # if fails, you should probably update the `prepare_inputs_for_generation` function