BPE save compatibility tweak

tokenizers/src/models/bpe/model.rs

@@ -444,6 +444,7 @@ impl Model for BPE {
             .map(|(pair, (rank, _))| (pair, rank))
             .collect();
         merges.sort_unstable_by_key(|k| *k.1);
+        merges_file.write_all(b"# Trained by `huggingface/tokenizers`\n")?:
         merges_file.write_all(
             &merges
                 .into_iter()