title: How to format requirements.txt when package source is from specific websites?
body: <p>I am trying to convert the following installation commands using pip that downloads from another website, into a requirements.txt format, but just can't figure out how. Can anyone assist?</p>
  
  <pre><code>pip install torch==1.5.0+cu101 torchvision==0.6.0+cu101 -f https://download.pytorch.org/whl/torch_stable.html
  
  pip install detectron2 -f https://dl.fbaipublicfiles.com/detectron2/wheels/cu101/index.html
  </code></pre>