Add an initial Segformer implementation (#1617)

* add segformer

* Make the id2label field optional.

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

candle-transformers/src/models/mod.rs

@@ -42,6 +42,7 @@ pub mod repvgg;
 pub mod resnet;
 pub mod rwkv_v5;
 pub mod rwkv_v6;
+pub mod segformer;
 pub mod segment_anything;
 pub mod stable_diffusion;
 pub mod stable_lm;