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.

candle-flash-attn/src/ffi.rs

@@ -0,0 +1,35 @@
+use core::ffi::{c_int, c_void};
+
+extern "C" {
+    pub(crate) fn run_mha(
+        q_ptr: *const c_void,
+        k_ptr: *const c_void,
+        v_ptr: *const c_void,
+        o_ptr: *const c_void,
+
+        q_batch_stride: u32,
+        k_batch_stride: u32,
+        v_batch_stride: u32,
+        q_row_stride: u32,
+        k_row_stride: u32,
+        v_row_stride: u32,
+        q_head_stride: u32,
+        k_head_stride: u32,
+        v_head_stride: u32,
+
+        b: u32,
+        h: u32,
+        h_k: u32,
+        d: u32,
+        d_rounded: u32,
+        softmax_scale: f32,
+
+        seqlen_q: u32,
+        seqlen_k: u32,
+        seqlen_q_rounded: u32,
+        seqlen_k_rounded: u32,
+
+        is_causal: c_int,
+    );
+
+}