use from_blob to avoid memcpy (#4118)

Co-authored-by: Nicolas Hug <nicolashug@fb.com>
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

torchvision/csrc/io/image/cpu/encode_jpeg.cpp

@@ -18,12 +18,12 @@ using namespace detail;
 
 torch::Tensor encode_jpeg(const torch::Tensor& data, int64_t quality) {
   // Define compression structures and error handling
-  struct jpeg_compress_struct cinfo;
-  struct torch_jpeg_error_mgr jerr;
+  struct jpeg_compress_struct cinfo {};
+  struct torch_jpeg_error_mgr jerr {};
 
   // Define buffer to write JPEG information to and its size
   unsigned long jpegSize = 0;
-  uint8_t* jpegBuf = NULL;
+  uint8_t* jpegBuf = nullptr;
 
   cinfo.err = jpeg_std_error(&jerr.pub);
   jerr.pub.error_exit = torch_jpeg_error_exit;
@@ -34,7 +34,7 @@ torch::Tensor encode_jpeg(const torch::Tensor& data, int64_t quality) {
      * We need to clean up the JPEG object and the buffer.
      */
     jpeg_destroy_compress(&cinfo);
-    if (jpegBuf != NULL) {
+    if (jpegBuf != nullptr) {
       free(jpegBuf);
     }
 
@@ -92,16 +92,10 @@ torch::Tensor encode_jpeg(const torch::Tensor& data, int64_t quality) {
   jpeg_destroy_compress(&cinfo);
 
   torch::TensorOptions options = torch::TensorOptions{torch::kU8};
-  auto outTensor = torch::empty({(long)jpegSize}, options);
-
-  // Copy memory from jpeg buffer, since torch cannot get ownership of it via
-  // `from_blob`
-  auto outPtr = outTensor.data_ptr<uint8_t>();
-  std::memcpy(outPtr, jpegBuf, sizeof(uint8_t) * outTensor.numel());
-
-  free(jpegBuf);
-
-  return outTensor;
+  auto out_tensor =
+      torch::from_blob(jpegBuf, {(long)jpegSize}, ::free, options);
+  jpegBuf = nullptr;
+  return out_tensor;
 }
 #endif