Do we need to build ffmpeg with cuda support in order to use GPU based decoding ?

Hi Decord Team,
Do we need to build ffmpeg with CUDA support as outlined in the Nvidia Official docs : [FFMPEG with CUDA (https://docs.nvidia.com/video-technologies/video-codec-sdk/ffmpeg-with-nvidia-gpu/index.html) in orer to use GPU based Decoding in Decord.
In the official install instructions for Decord I see that we straightway install the latest ffmpeg without compiling from source
````
# official PPA comes with ffmpeg 2.8, which lacks tons of features, we use ffmpeg 4.0 here
sudo add-apt-repository ppa:jonathonf/ffmpeg-4 # for ubuntu20.04 official PPA is already version 4.2, you may skip this step
sudo apt-get update
sudo apt-get install -y build-essential python3-dev python3-setuptools make cmake
sudo apt-get install -y ffmpeg libavcodec-dev libavfilter-dev libavformat-dev libavutil-dev
# note: make sure you have cmake 3.8 or later, you can install from cmake official website if it's too old
````

Is there a script or a way to ascertain that we are using GPU based decoding (instead of CPU based) in Decord ?

Start with the Decord installation instructions and the linked NVIDIA FFMPEG with CUDA documentation. Determine whether the documented FFmpeg installation supports Decord's GPU decoding and how users can verify which decoding path is active. Done means providing a clear answer or updating the relevant installation documentation.

Written by the indexing model from the issue text.