Use version.txt to determine torchvision version in CMakeLists.txt (#3381)

Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

CMakeLists.txt

 cmake_minimum_required(VERSION 3.12)
 project(torchvision)
 set(CMAKE_CXX_STANDARD 14)
-set(TORCHVISION_VERSION 0.7.0)
+file(STRINGS version.txt TORCHVISION_VERSION)
 
 option(WITH_CUDA "Enable CUDA support" OFF)