getFrameAtIndexInternal now *must* take preAllocatedTensor

Author: NicolasHug

src/torchcodec/decoders/_core/VideoDecoder.cpp

@@ -1032,7 +1032,7 @@ VideoDecoder::DecodedOutput VideoDecoder::getFrameAtIndex(
 VideoDecoder::DecodedOutput VideoDecoder::getFrameAtIndexInternal(
     int streamIndex,
     int64_t frameIndex,
-    std::optional<torch::Tensor> preAllocatedOutputTensor) {
+    torch::Tensor preAllocatedOutputTensor) {
   validateUserProvidedStreamIndex(streamIndex);
   validateScannedAllStreams("getFrameAtIndex");
 

src/torchcodec/decoders/_core/VideoDecoder.h

@@ -232,7 +232,7 @@ class VideoDecoder {
   DecodedOutput getFrameAtIndexInternal(
       int streamIndex,
       int64_t frameIndex,
-      std::optional<torch::Tensor> preAllocatedOutputTensor = std::nullopt);
+      torch::Tensor preAllocatedOutputTensor);
 
   struct BatchDecodedOutput {
     torch::Tensor frames;