Add the Mistral 7b model (#983)

* Start sketching the mistral 7b model.

* Add the kv cache.

* Add the decoder layer.

* Add the mistral model.

* Rotary embeddings.

* Add the attention mask.

candle-transformers/src/models/mod.rs

@@ -4,6 +4,7 @@ pub mod dinov2;
 pub mod efficientnet;
 pub mod falcon;
 pub mod llama;
+pub mod mistral;
 pub mod mixformer;
 pub mod quantized_llama;
 pub mod quantized_mixformer;