Fix looking up OOVs in lexicon.txt for MeloTTS models.

If an English word does not exist in the lexicon, we split
it into characters. For instance, if the word TTS does not
exist in lexicon.txt, we split it into 3 characters T, T, and S.

sherpa-onnx/csrc/melo-tts-lexicon.cc

@@ -136,6 +136,22 @@ class MeloTtsLexicon::Impl {
         ans.tokens.insert(ans.tokens.end(), ids.tokens.begin(),
                           ids.tokens.end());
         ans.tones.insert(ans.tones.end(), ids.tones.begin(), ids.tones.end());
+      } else {
+        // If the lexicon does not contain the word, we split the word into
+        // characters.
+        //
+        // For instance, if the word is TTS and it is does not exist
+        // in the lexicon, we split it into 3 characters: T T S
+        std::string s;
+        for (char c : word) {
+          s = c;
+          if (word2ids_.count(s)) {
+            const auto &t = word2ids_.at(s);
+            ans.tokens.insert(ans.tokens.end(), t.tokens.begin(),
+                              t.tokens.end());
+            ans.tones.insert(ans.tones.end(), t.tones.begin(), t.tones.end());
+          }
+        }
       }
     }