Fixed some ffmpeg deprecation warnings in decoder (#4003)

* Fixed some ffmpeg deprecation warnings in decoder

* Fixed clang error
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

torchvision/csrc/io/decoder/decoder.cpp

 #include "decoder.h"
 #include <c10/util/Logging.h>
+#include <libavutil/avutil.h>
 #include <future>
 #include <iostream>
 #include <mutex>
@@ -196,8 +197,10 @@ int64_t Decoder::seekCallback(int64_t offset, int whence) {
 void Decoder::initOnce() {
   static std::once_flag flagInit;
   std::call_once(flagInit, []() {
+#if LIBAVUTIL_VERSION_MAJOR < 56 // Before FFMPEG 4.0
     av_register_all();
     avcodec_register_all();
+#endif
     avformat_network_init();
     // register ffmpeg lock manager
     av_lockmgr_register(&ffmpeg_lock);
@@ -397,10 +400,14 @@ bool Decoder::init(
 }
 
 bool Decoder::openStreams(std::vector<DecoderMetadata>* metadata) {
-  for (int i = 0; i < inputCtx_->nb_streams; i++) {
+  for (unsigned int i = 0; i < inputCtx_->nb_streams; i++) {
     // - find the corespondent format at params_.formats set
     MediaFormat format;
+#if LIBAVUTIL_VERSION_MAJOR < 56 // Before FFMPEG 4.0
     const auto media = inputCtx_->streams[i]->codec->codec_type;
+#else // FFMPEG 4.0+
+    const auto media = inputCtx_->streams[i]->codecpar->codec_type;
+#endif
     if (!mapFfmpegType(media, &format.type)) {
       VLOG(1) << "Stream media: " << media << " at index " << i
               << " gets ignored, unknown type";