The GPT-2 based Musenet music generator is already interesting but far from perfect. You can try it in the middle of this article: https://openai.com/blog/musenet/ (you can even upload custom prompts in the advanced mode) Would be interesting to see it with the updated GPT-3.

There is also AIVA with more production ready results:

https://www.youtube.com/watch?v=gzGkC_o9hXI&list=PLv7BOfa4Cx...

Not sure how it works, but it has better results maybe because it's using more predefined components and less AI so it's also less "creative".

More AI music projects here: https://magenta.tensorflow.org/