1.9.0a0 is a pre-1.9.0 release and thus not the latest version.
In any case, this commit works for me using CUDA 11.2:
git clone https://github.com/pytorch/pytorch
cd pytorch
git checkout dfbd030
git submodule update --init --recursive 
python setup.py install

>>> import torch
>>> torch.__version__
'1.9.0a0+df837d0'
>>> torch.version.cuda
'11.2'