Optimize the logit computations in the whisper example. (#434)

candle-examples/examples/whisper/main.rs

@@ -10,7 +10,7 @@ extern crate accelerate_src;
 extern crate intel_mkl_src;
 
 use anyhow::{Error as E, Result};
-use candle::{DType, Device, Tensor};
+use candle::{DType, Device, IndexOp, Tensor};
 use candle_nn::{ops::softmax, VarBuilder};
 use clap::{Parser, ValueEnum};
 use hf_hub::{api::sync::Api, Repo, RepoType};
@@ -128,21 +128,24 @@ impl Decoder {
             // The model expects a batch dim but this inference loop does not handle
             // it so we add it at this point.
             let tokens_t = tokens_t.unsqueeze(0)?;
-            let logits = model.decoder.forward(&tokens_t, &audio_features, i == 0)?;
-            let logits = logits.squeeze(0)?;
+            let ys = model.decoder.forward(&tokens_t, &audio_features, i == 0)?;
 
             // Extract the no speech probability on the first iteration by looking at the first
             // token logits and the probability for the according token.
             if i == 0 {
-                no_speech_prob = softmax(&logits.get(0)?, 0)?
-                    .get(self.no_speech_token as usize)?
+                let logits = model.decoder.final_linear(&ys.i(..1)?)?.i(0)?.i(0)?;
+                no_speech_prob = softmax(&logits, 0)?
+                    .i(self.no_speech_token as usize)?
                     .to_scalar::<f32>()? as f64;
             }
 
-            let (seq_len, _) = logits.dims2()?;
-            let logits = logits
-                .get(seq_len - 1)?
-                .broadcast_add(&self.suppress_tokens)?;
+            let (_, seq_len, _) = ys.dims3()?;
+            let logits = model
+                .decoder
+                .final_linear(&ys.i((..1, seq_len - 1..))?)?
+                .i(0)?
+                .i(0)?;
+            let logits = logits.broadcast_add(&self.suppress_tokens)?;
             let next_token = if t > 0f64 {
                 let prs = softmax(&(&logits / t)?, 0)?;
                 let logits_v: Vec<f32> = prs.to_vec1()?;
@@ -159,7 +162,7 @@ impl Decoder {
             };
             tokens.push(next_token);
             let prob = softmax(&logits, candle::D::Minus1)?
-                .get(next_token as usize)?
+                .i(next_token as usize)?
                 .to_scalar::<f32>()? as f64;
             if next_token == self.eot_token || tokens.len() > model.config.max_target_positions {
                 break;

candle-examples/examples/whisper/model.rs

@@ -364,11 +364,13 @@ pub struct TextDecoder {
     ln: LayerNorm,
     mask: Tensor,
     span: tracing::Span,
+    span_final: tracing::Span,
 }
 
 impl TextDecoder {
     fn load(vb: VarBuilder, cfg: &Config) -> Result<Self> {
         let span = tracing::span!(tracing::Level::TRACE, "text-decoder");
+        let span_final = tracing::span!(tracing::Level::TRACE, "text-decoder-final");
         let n_state = cfg.d_model;
         let n_head = cfg.decoder_attention_heads;
         let n_ctx = cfg.max_target_positions;
@@ -384,7 +386,6 @@ impl TextDecoder {
             .flat_map(|i| (0..n_ctx).map(move |j| if j > i { f32::NEG_INFINITY } else { 0f32 }))
             .collect();
         let mask = Tensor::from_vec(mask, (n_ctx, n_ctx), vb.device())?;
-
         Ok(Self {
             token_embedding,
             positional_embedding,
@@ -392,6 +393,7 @@ impl TextDecoder {
             ln,
             mask,
             span,
+            span_final,
         })
     }
 
@@ -405,12 +407,16 @@ impl TextDecoder {
         for block in self.blocks.iter_mut() {
             x = block.forward(&x, Some(xa), Some(&self.mask), flush_kv_cache)?;
         }
-        let x = self.ln.forward(&x)?;
-        let w = self
-            .token_embedding
-            .embeddings()
-            .broadcast_left(x_dims[0])?;
-        let logits = x.matmul(&w.t()?)?;
+        self.ln.forward(&x)
+    }
+
+    pub fn final_linear(&self, x: &Tensor) -> Result<Tensor> {
+        let b_size = x.dim(0)?;
+        let w = self.token_embedding.embeddings().broadcast_left(b_size)?;
+        let logits = {
+            let _enter = self.span_final.enter();
+            x.matmul(&w.t()?)?
+        };
         Ok(logits)
     }
 }
@@ -432,16 +438,4 @@ impl Whisper {
             config,
         })
     }
-
-    #[allow(dead_code)]
-    pub fn forward(
-        &mut self,
-        mel: &Tensor,
-        tokens: &Tensor,
-        flush_kv_cache: bool,
-    ) -> Result<Tensor> {
-        let enc = self.encoder.forward(mel, flush_kv_cache)?;
-        let dec = self.decoder.forward(tokens, &enc, flush_kv_cache)?;
-        Ok(dec)
-    }
 }