GPT-2 was developed by Open-AI. They recently switched to Pytorch themselves for similar reasons [1]. I didn't claim that
all research that matters is happening on Pytorch, but it is very true that most non-Google research is [2]. Having used both, Pytorch has been a far better experience for me, and lately, most of the reference implementations I care about are released in Pytorch, so it makes it very easy to experiment with and also pull into my own code base if needed.
Also, for an intro class, Pytorch would have been a better choice imo, because it has a simpler and cleaner API. In my experience, it was a lot easier to get up and running with Pytorch compared to Tensorflow.
1 - https://openai.com/blog/openai-pytorch/
2 - https://news.ycombinator.com/item?id=21216200