I cannot reproduce this issue and am able to import the mentioned modules:
>>> import torch
>>> import torchvision
>>> torch.__version__
'1.7.1'
>>> torchvision.__version__
'0.8.2'
>>> import torchvision.models as models
>>> models.detection
<module 'torchvision.models.detection' from '.../anaconda3/envs/pytorch_171/lib/python3.8/site-packages/torchvision/models/detection/__init__.py'>
>>> torchvision.ops
<module 'torchvision.ops' from '.../anaconda3/envs/pytorch_171/lib/python3.8/site-packages/torchvision/ops/__init__.py'>

Are you sure you’ve installed the latest PyTorch and torchvision releases?