2d convolve performance improvements

changed the shared memory loading access pattern in 2d convolve
kernel for cuda and opencl backends
Merge pull request #227 from 9prady9/conv2d_perf_fixes

2d convolve performance improvements
Merge pull request #229 from shehzan10/devel

Added pinned memory creation
Updating CONTRIBUTING.md
Added warning messages for when submodules are not cloned