raspberry3 install torch1.7.0 python3.7 whl

1. download torch1.7.0 whl：https://github.com/ljk53/pytorch-rpi/blob/master/torch-1.7.0a0-cp37-cp37m-linux_armv7l.whl

2. upload the whl to the raspberry pi

3. Pi terminal: pip3 install torch-1.7.0a0-cp37-cp37m-linux_armv7l.whl