[fix] Prevent IndexError if output_hidden_states & ONNX

[tomaarsen]

Resolves #3004

Hello!

Pull Request overview

• Prevent IndexError if output_hidden_states & ONNX

Details

Some models specify output_hidden_states in their config, but the ONNX model doesn't respect this. As a result, the output_states is just 1 value: the last_hidden_states, whereas the code expects that there's also a all_hidden_states value.

Now we ensure that there's at least 2 outputs to prevent a crash.

Code

from sentence_transformers import SentenceTransformer

model = SentenceTransformer(
    "distiluse-base-multilingual-cased",
    backend="onnx",
    model_kwargs={"provider": "CPUExecutionProvider"},
)

sentences = ["This is an example sentence", "Each sentence is converted"]
embeddings = model.encode(sentences)
print(embeddings.shape)

This script used to fail, see #3004.

• Tom Aarsen