Add the RWKV model (v5). (#1707)

* Start adding the RWKV model.

* More of the forward step.

* Handle rescaling.

* FeedForward.

* More work on RWKV.

* Better state tracking.

* Finish a first pass on forward.

* Fix the shape mismatches.

* Do not rescale in f32.

* Rename to rwkv-v5.

* Add the new models to the readme.

candle-transformers/src/models/mod.rs

@@ -34,6 +34,7 @@ pub mod quantized_t5;
 pub mod qwen2;
 pub mod repvgg;
 pub mod resnet;
+pub mod rwkv_v5;
 pub mod segment_anything;
 pub mod stable_diffusion;
 pub mod stable_lm;