I can install PyTorch 1.7.0 with the CUDA10.2 runtime in a new Colab notebook.
After creating a new notebook, it seems 1.6 with CUDA10.1 is installed.
Uninstalling this wheel and installing the new one via:
!pip uninstall torch torchvision -y
!pip install torch torchvision

results in:
import torch
print(torch.__version__)
> 1.7.0
print(torch.version.cuda)
> 10.2