Improve the sampling methods.

candle-transformers/src/generation/mod.rs

@@ -1,4 +1,4 @@
-use candle::{DType, Error, Result, Tensor};
+use candle::{DType, Error, IndexOp, Result, Tensor, D};
 use rand::{distributions::Distribution, SeedableRng};
 
 #[derive(Clone, PartialEq, Debug)]
@@ -73,17 +73,15 @@ impl LogitsProcessor {
     }
 
     // top-k sampling samples from the k tokens with the largest probabilities.
-    fn sample_topk(&mut self, prs: &mut Vec<f32>, top_k: usize) -> Result<u32> {
-        if top_k >= prs.len() {
-            self.sample_multinomial(prs)
-        } else {
-            let mut argsort_indices = (0..prs.len()).collect::<Vec<_>>();
-            let (indices, _, _) =
-                argsort_indices.select_nth_unstable_by(top_k, |&i, &j| prs[j].total_cmp(&prs[i]));
-            let prs = indices.iter().map(|&i| prs[i]).collect::<Vec<_>>();
-            let index = self.sample_multinomial(&prs)?;
-            Ok(indices[index as usize] as u32)
-        }
+    fn sample_topk(&mut self, logits: &Tensor, top_k: usize, temperature: f64) -> Result<u32> {
+        let arg_sort = logits.arg_sort_last_dim(false)?;
+        let top_k_indices = arg_sort.narrow(candle::D::Minus1, 0, top_k)?;
+        let top_k_logits = logits.gather(&top_k_indices, D::Minus1)?;
+        let top_k_logits = (&top_k_logits / temperature)?;
+        let top_k_prs = candle_nn::ops::softmax_last_dim(&top_k_logits)?;
+        let top_k_prs = top_k_prs.to_vec1()?;
+        let index = self.sample_multinomial(&top_k_prs)?;
+        Ok(top_k_indices.i(index as usize)?.to_vec0::<u32>()?)
     }
 
     // top-k sampling samples from the k tokens with the largest probabilities.
@@ -137,8 +135,12 @@ impl LogitsProcessor {
                 }
             }
             Sampling::TopK { k, temperature } => {
-                let mut prs = prs(*temperature)?;
-                self.sample_topk(&mut prs, *k)?
+                if *k >= logits.dim(D::Minus1)? {
+                    let prs = prs(*temperature)?;
+                    self.sample_multinomial(&prs)?
+                } else {
+                    self.sample_topk(&logits, *k, *temperature)?
+                }
             }
             Sampling::TopKThenTopP { k, p, temperature } => {
                 let mut prs = prs(*temperature)?;