Fix duplicated memory allocation in StreamWriter (#2906)

Summary:
Pull Request resolved: #2906

The correct way to create AVFormatContext* for output is to pass an address of an uninitialized *AVFormatContext struct to `avformat_alloc_output_context2` function.

The current code pre-allocates AVFormatContext* with `avformat_alloc_context`, then this allocated object is lost inside of `avformat_alloc_output_context2`.

Reviewed By: xiaohui-zhang

Differential Revision: D41865685

fbshipit-source-id: 9a9dc83b5acfe9b450f191fe716c85ebb5a5d842

Author: mthrok

torchaudio/csrc/ffmpeg/stream_writer/stream_writer_wrapper.cpp

@@ -13,9 +13,7 @@ AVFormatOutputContextPtr get_output_format_context(
         "`format` must be provided when the input is file-like object.");
   }
 
-  AVFormatContext* p = avformat_alloc_context();
-  TORCH_CHECK(p, "Failed to allocate AVFormatContext.");
-
+  AVFormatContext* p = nullptr;
   int ret = avformat_alloc_output_context2(
       &p, nullptr, format ? format.value().c_str() : nullptr, dst.c_str());
   TORCH_CHECK(