python3 setup.py build develop but No module named 'torch',HELP!!

[pengpengdesu]

when i input "python3 setup.py build develop"
error happend:
error: can't create or remove files in install directory
The following error occurred while trying to add or remove files in the
installation directory:
[Errno 13] Permission denied: '/usr/local/lib/python3.8/dist-packages/test-easy-install-18392.write-test'
The installation directory you specified (via --install-dir, --prefix, or
the distutils default setting) was:
/usr/local/lib/python3.8/dist-packages/

but when i input "sudo python3 setup.py build develop"
error is
Traceback (most recent call last):
File "setup.py", line 7, in
import torch
ModuleNotFoundError: No module named 'torch'

however i have already installed pytorch
ghy@ghy:~/disktop/faster-rcnn.pytorch$ python3
Python 3.8.10 (default, Nov 26 2021, 20:14:08)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

import torch
torch.version
'1.8.1+cu111'