Add CTC HLG decoding using OpenFst (#349)

sherpa-onnx/csrc/offline-ctc-model.h

@@ -6,7 +6,7 @@
 
 #include <memory>
 #include <string>
-#include <utility>
+#include <vector>
 
 #if __ANDROID_API__ >= 9
 #include "android/asset_manager.h"
@@ -32,17 +32,17 @@ class OfflineCtcModel {
 
   /** Run the forward method of the model.
    *
-   * @param features  A tensor of shape (N, T, C). It is changed in-place.
+   * @param features  A tensor of shape (N, T, C).
    * @param features_length  A 1-D tensor of shape (N,) containing number of
    *                         valid frames in `features` before padding.
    *                         Its dtype is int64_t.
    *
-   * @return Return a pair containing:
+   * @return Return a vector containing:
    *  - log_probs: A 3-D tensor of shape (N, T', vocab_size).
    *  - log_probs_length A 1-D tensor of shape (N,). Its dtype is int64_t
    */
-  virtual std::pair<Ort::Value, Ort::Value> Forward(
-      Ort::Value features, Ort::Value features_length) = 0;
+  virtual std::vector<Ort::Value> Forward(Ort::Value features,
+                                          Ort::Value features_length) = 0;
 
   /** Return the vocabulary size of the model
    */