Add flash attention (#241)

* Add some flash-attn kernel, import the code for flash-attn v2 from Dao-AILab.

* More flash attn.

* Set up the flash attn parameters.

* Get things to compile locally.

* Move the flash attention files in a different directory.

* Build the static C library with nvcc.

* Add more flash attention.

* Update the build part.

* Better caching.

* Exclude flash attention from the default workspace.

* Put flash-attn behind a feature gate.

* Get the flash attn kernel to run.

* Move the flags to a more appropriate place.

* Enable flash attention in llama.

* Use flash attention in llama.

Cargo.toml

@@ -9,6 +9,7 @@ members = [
     "candle-wasm-examples/whisper",
 ]
 exclude = [
+    "candle-flash-attn",
     "candle-kernels",
 ]