Extract NVDEC tutorial from the current notebook (#3478)

Summary:
Now that GPU video decoders are available in doc CI, we run the tutorials with GPU decoders.

Pull Request resolved: https://github.com/pytorch/audio/pull/3478

Differential Revision: D47519672

Pulled By: mthrok

fbshipit-source-id: 2f95243100e9c75e17c2b4d306da164f0e31f8f2

docs/source/build.ffmpeg.rst

+.. _enabling_hw_decoder:
+
 Enabling GPU video decoder/encoder
 ==================================
 
@@ -468,28 +470,4 @@ Using the hardware decoder
 
 Once the installation and the runtime linking work fine, then you can test the GPU decoding with the following.
 
-For the detail on the performance of GPU decoder and encoder please see `Hardware-Accelerated Video Decoding and Encoding <./hw_acceleration_tutorial.html>`_
-
-
-.. code-block:: python
-
-   from torchaudio.io import StreamReader
-
-   src = "https://download.pytorch.org/torchaudio/tutorial-assets/stream-api/NASAs_Most_Scientifically_Complex_Space_Observatory_Requires_Precision-MP4_small.mp4"
-
-   s = StreamReader(src)
-   s.add_video_stream(
-       5,
-       decoder="h264_cuvid",
-       hw_accel="cuda:0",
-       decoder_option={
-           "resize": "360x240",
-       },
-   )
-   s.fill_buffer()
-   chunk, = s.pop_chunks()
-   print(' - Chunk:', chunk.shape, chunk.device, chunk.dtype)
-
-.. code-block:: text
-
-   - Chunk: torch.Size([5, 3, 240, 360]) cuda:0 torch.uint8
+For the detail on the performance of GPU decoder please see :ref:`NVDEC tutoial <nvdec_tutorial>`.

docs/source/index.rst

@@ -45,6 +45,7 @@ model implementations and application components.
    tutorials/streamwriter_basic_tutorial
    tutorials/streamwriter_advanced
    hw_acceleration_tutorial
+   tutorials/nvdec_tutorial
 
    tutorials/effector_tutorial
    tutorials/audio_resampling_tutorial
@@ -194,6 +195,13 @@ Tutorials
    :link: tutorials/streamwriter_advanced.html
    :tags: I/O,StreamWriter
 
+.. customcarditem::
+   :header: Hardware accelerated video decoding with NVDEC
+   :card_description: Learn how to use HW video decoder.
+   :image: https://download.pytorch.org/torchaudio/tutorial-assets/thumbnails/hw_acceleration_tutorial.png
+   :link: tutorials/nvdec_tutorial.html
+   :tags: I/O,StreamReader
+
 .. customcarditem::
    :header: Hardware accelerated video I/O with NVDEC/NVENC
    :card_description: Learn how to setup and use HW accelerated video I/O.