Thanks for your kind help. I use the old command
conda install pytorch=1.0.0 torchvision cuda100 -c pytorch

and with pip install fastai install the pytorch and fastai successfully.
Even though I am not fully understanding the reason why it is cudnn 7.4, but the test result is right.
I test the environment with a mnist example. and here I just show the versions of them

In [2]: import fastai

In [3]: print(torch.__version__)
1.0.0

In [4]: print(fastai.__version__)
1.0.51

And I know the format of my question is very bad but you’d still love to help me our.
Thanks a lot!
Have a good day!