Add the recurrent-gemma model. (#2039)

* Start adding the recurrent-gemma model.

* More griffin.

* Add the example + get the weights to load from the HF version.

* More inference code.

* Rope + kv-cache on the attention side.

* Add to the inference code.

* Add more to the recurrent gemma inference.

* Get some first inference to run.

* Add the softcap on logits.

* Fixes.

* Use partial rotary embeddings.

* Get inference to work.

* Add a comment.

* And add a readme.

candle-examples/examples/recurrent-gemma/README.md

@@ -0,0 +1,9 @@
+# candle-recurrent-gemma
+
+This model card corresponds to the 2B base version of the RecurrentGemma model
+[huggingface model card](https://huggingface.co/google/recurrentgemma-2b).
+
+```bash
+cargo run --features cuda -r --example recurrent-gemma -- \
+    --prompt "Write me a poem about Machine Learning."  
+```