Encoder model support for the Transformers backend (#25174)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/models/test_transformers.py

@@ -9,7 +9,7 @@ from vllm.platforms import current_platform
 
 from ..conftest import HfRunner, VllmRunner
 from ..utils import multi_gpu_test, prep_prompts
-from .utils import check_logprobs_close
+from .utils import check_embeddings_close, check_logprobs_close
 
 
 def check_implementation(
@@ -165,6 +165,40 @@ def test_embed_loading(vllm_runner, model):
         assert model_config.using_transformers_backend()
 
 
+@pytest.mark.parametrize(
+    "model",
+    [
+        # Encoder model
+        "BAAI/bge-base-en-v1.5",
+    ])
+def test_embed_correctness(hf_runner, vllm_runner, example_prompts, model):
+    import transformers
+    from packaging.version import Version
+    installed = Version(transformers.__version__)
+    required = Version("4.57.0.dev0")
+    if installed < required:
+        pytest.skip("Encoder models with the Transformers backend require "
+                    f"transformers>={required}, but got {installed}")
+
+    with vllm_runner(model, max_model_len=512,
+                     model_impl="transformers") as vllm_model:
+        model_config = vllm_model.llm.llm_engine.model_config
+        assert model_config.using_transformers_backend()
+
+        vllm_outputs = vllm_model.embed(example_prompts)
+
+    with hf_runner(model, is_sentence_transformer=True) as hf_model:
+        hf_outputs = hf_model.encode(example_prompts)
+
+    check_embeddings_close(
+        embeddings_0_lst=hf_outputs,
+        embeddings_1_lst=vllm_outputs,
+        name_0="hf",
+        name_1="vllm",
+        tol=1e-2,
+    )
+
+
 @pytest.mark.parametrize(
     "model",
     ["jason9693/Qwen2.5-1.5B-apeach"],