Add exported onnx model 'model.onnx'

Hello!

*This pull request has been automatically generated from the [Sentence Transformers backend-export](https://huggingface.co/spaces/sentence-transformers/backend-export) Space.*

## Pull Request overview
* Add exported ONNX model `model.onnx`.

## Tip:
Consider testing this pull request before merging by loading the model from this PR with the `revision` argument:
```python
from sentence_transformers import SentenceTransformer

# TODO: Fill in the PR number
pr_number = 2
model = SentenceTransformer(
"marquesafonso/albertina-sts",
revision=f"refs/pr/{pr_number}",
backend="onnx",
)

# Verify that everything works as expected
embeddings = model.encode(["The weather is lovely today.", "It's so sunny outside!", "He drove to the stadium."])
print(embeddings.shape)

similarities = model.similarity(embeddings, embeddings)
print(similarities)
```

onnx/model.onnx

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:bd7416789694bcd4d937667bfde430df5cbe61c347a5055b1591e751d7812285
+size 554923490