Remove lazy conv support
- upstream has changed to new convolution_overrideable +
convolution_backward_overrideable signatures, which should help lazy
- but these are not structured kernels, so we have to port the shape
inference functions
- the existing lazy conv impls can't continue to compile since conv
wrappers they call have disappeared during the upstream refactor
So, we need to re-implement conv for lazy using the new convs, and
do this separately from this merge.
