Fix rewind crash

crates/llm-base/src/inference_session.rs

@@ -360,7 +360,7 @@ impl InferenceSession {
  }
 
  // Remove the tokens from self.tokens.
- let token_start = self.n_past - num;
+ let token_start = self.tokens.len() - num;
  let deleted_tokens: Vec<_> = self.tokens.drain(token_start..).collect();
 
  // Remove the corresponding chars from decoded