feat: Add cross_encoder serving and fix text_classification token_type_ids

[georgeguimaraes]

I was experimenting with rerankers for georgeguimaraes/arcana and found that cross-encoder models like cross-encoder/ms-marco-MiniLM-L-6-v2 weren't producing correct scores.

The issue: text_classification was setting return_token_type_ids: false, which breaks sentence-pair inputs. Cross-encoders need token_type_ids to distinguish query tokens from document tokens. Without them, scores don't match Python's sentence-transformers.

Changes:

• Fixed text_classification to include token_type_ids (also added it to the compile template)
• Added a new cross_encoder serving with a cleaner API for the reranking use case

serving = Bumblebee.Text.cross_encoder(model_info, tokenizer)
Nx.Serving.run(serving, {"query", "document"})
#=> %{score: 8.76}

The token_type_ids fix also benefits other sentence-pair tasks like NLI and entailment. If you don't want that change in text_classification, let me know.

Closes #251

[Copilot]

Pull request overview

This PR adds cross-encoder support and fixes token_type_ids handling for sentence-pair classification tasks. Cross-encoder models like cross-encoder/ms-marco-MiniLM-L-6-v2 require token_type_ids to distinguish query tokens from document tokens, which were previously disabled in text_classification.

Changes:

• Fixed text_classification to include token_type_ids for sentence-pair inputs
• Added new cross_encoder serving with a dedicated API for reranking use cases
• Added comprehensive test coverage for both changes

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/bumblebee/text/text_classification.ex	Removed return_token_type_ids: false configuration and added token_type_ids to compile template
lib/bumblebee/text/cross_encoder.ex	New module implementing cross-encoder serving with pair validation and score extraction
lib/bumblebee/text.ex	Added cross_encoder function documentation, type specs, and public API delegation
test/bumblebee/text/text_classification_test.exs	Added test verifying correct scoring for cross-encoder sentence pairs
test/bumblebee/text/cross_encoder_test.exs	New test file with comprehensive coverage for single and batch pair scoring

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[jonatanklosko]

Just one note about naming and looks good to me!

[jonatanklosko]

Nit: let's rename to "cross encoding" to match all the other functions we already have (embeding, classifiction, etc):

Suggested change

	defdelegate cross_encoder(model_info, tokenizer, opts \\ []),
	to: Bumblebee.Text.CrossEncoder
	defdelegate cross_encoding(model_info, tokenizer, opts \\ []),
	to: Bumblebee.Text.CrossEncoding

[georgeguimaraes]

done