Fix the pytorch version

docs/installation.md

@@ -90,7 +90,7 @@ The code has been tested with Ubuntu 16.04 and PyTorch 1.6.0.
    ```shell
    conda create --name pytorch-1.6.0 python=3.7
    conda activate pytorch-1.6.0
-   conda install pytorch torchvision cudatoolkit=10.1 -c pytorch
+   conda install pytorch==1.6.0 torchvision==0.7.0 cudatoolkit=10.1 -c pytorch
    conda install tqdm yacs -c conda-forge
    ```
 2. Build O-CNN under PyTorch.