Added XLMRobertaModel for Reranking (#2686)

* add xlm-roberta-base

* Add task enum for fill-mask and reranker in xlm-roberta example; update README and fix attention mask dimensions

- Introduced a new `Task` enum to replace string task identifiers in the xlm-roberta example.
- Updated the logic in `main.rs` to handle tasks using the new enum.
- Enhanced README with example output for fill-mask task.
- Fixed dimension retrieval in `prepare_4d_attention_mask` function for better clarity and safety.

* Clippy fix.

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Co-authored-by: laurent <laurent.mazare@gmail.com>

candle-transformers/src/models/mod.rs

@@ -109,4 +109,5 @@ pub mod vit;
 pub mod whisper;
 pub mod with_tracing;
 pub mod wuerstchen;
+pub mod xlm_roberta;
 pub mod yi;