Hello Eugene Huang
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
You are correct that the NVIDIA GPU Driver Extension for Linux only supports Red Hat Enterprise Linux 7.3, 7.4, 7.5, 7.6, 7.7, and 7.8**.** There is no mention of support for RHEL 8.x.
However, you can manually install the drivers on RHEL 8.x by following the instructions provided in this document and see if that works.
https://learn.microsoft.com/en-us/azure/virtual-machines/linux/n-series-driver-setup
Hope that helps.