Enforce that encode() cannot be called twice

Author: NicolasHug

src/torchcodec/_core/Encoder.cpp

@@ -208,9 +208,12 @@ torch::Tensor AudioEncoder::encodeToTensor() {
 }
 
 void AudioEncoder::encode() {
-  // TODO-ENCODING: Need to check, but consecutive calls to encode() are
-  // probably invalid. We can address this once we (re)design the public and
-  // private encoding APIs.
+  // To be on the safe side we enforce that encode() can only be called once on
+  // an encoder object. Whether this is actually necessary is unknown, so this
+  // may be relaxed if needed.
+  TORCH_CHECK(!encodeWasCalled_, "Cannot call encode() twice.");
+  encodeWasCalled_ = true;
+
   UniqueAVFrame avFrame(av_frame_alloc());
   TORCH_CHECK(avFrame != nullptr, "Couldn't allocate AVFrame.");
   //  Default to 256 like in torchaudio

src/torchcodec/_core/Encoder.h

@@ -49,5 +49,7 @@ class AudioEncoder {
 
   // Stores the AVIOContext for the output tensor buffer.
   std::unique_ptr<AVIOToTensorContext> avioContextHolder_;
+
+  bool encodeWasCalled_ = false;
 };
 } // namespace facebook::torchcodec