Streaming mode for reporting the generated tokens (#1007)

* Token streaming.

* Use the token output stream.

* Flush the output.

* Ensure that the last characters get reported.

candle-examples/src/token_output_stream.rs

@@ -0,0 +1,74 @@
+use candle::Result;
+
+/// This is a wrapper around a tokenizer to ensure that tokens can be returned to the user in a
+/// streaming way rather than having to wait for the full decoding.
+pub struct TokenOutputStream {
+    tokenizer: tokenizers::Tokenizer,
+    tokens: Vec<u32>,
+    prev_index: usize,
+    current_index: usize,
+}
+
+impl TokenOutputStream {
+    pub fn new(tokenizer: tokenizers::Tokenizer) -> Self {
+        Self {
+            tokenizer,
+            tokens: Vec::new(),
+            prev_index: 0,
+            current_index: 0,
+        }
+    }
+
+    pub fn into_inner(self) -> tokenizers::Tokenizer {
+        self.tokenizer
+    }
+
+    fn decode(&self, tokens: &[u32]) -> Result<String> {
+        match self.tokenizer.decode(tokens, true) {
+            Ok(str) => Ok(str),
+            Err(err) => candle::bail!("cannot decode: {err}"),
+        }
+    }
+
+    // https://github.com/huggingface/text-generation-inference/blob/5ba53d44a18983a4de32d122f4cb46f4a17d9ef6/server/text_generation_server/models/model.py#L68
+    pub fn next_token(&mut self, token: u32) -> Result<Option<String>> {
+        let prev_text = if self.tokens.is_empty() {
+            String::new()
+        } else {
+            let tokens = &self.tokens[self.prev_index..self.current_index];
+            self.decode(tokens)?
+        };
+        self.tokens.push(token);
+        let text = self.decode(&self.tokens[self.prev_index..])?;
+        if text.len() > prev_text.len() && text.chars().last().unwrap().is_ascii() {
+            let text = text.split_at(prev_text.len());
+            self.prev_index = self.current_index;
+            self.current_index = self.tokens.len();
+            Ok(Some(text.1.to_string()))
+        } else {
+            Ok(None)
+        }
+    }
+
+    pub fn decode_rest(&self) -> Result<String> {
+        self.decode(&self.tokens[self.prev_index..])
+    }
+
+    pub fn decode_all(&self) -> Result<String> {
+        self.decode(&self.tokens)
+    }
+
+    pub fn get_token(&self, token_s: &str) -> Option<u32> {
+        self.tokenizer.get_vocab(true).get(token_s).copied()
+    }
+
+    pub fn tokenizer(&self) -> &tokenizers::Tokenizer {
+        &self.tokenizer
+    }
+
+    pub fn clear(&mut self) {
+        self.tokens.clear();
+        self.prev_index = 0;
+        self.current_index = 0;
+    }
+}