Add the MMDiT model of Stable Diffusion 3 (#2397)

* add mmdit of stable diffusion 3

lint

add comments

* correct a misplaced comment

* fix cargo fmt

* fix clippy error

* use bail! instead of assert!

* use get_on_dim in splitting qkv

candle-transformers/src/models/mod.rs

@@ -32,6 +32,7 @@ pub mod metavoice;
 pub mod mistral;
 pub mod mixformer;
 pub mod mixtral;
+pub mod mmdit;
 pub mod mobilenetv4;
 pub mod mobileone;
 pub mod moondream;