`conv3d` operation missing about mlx

[awni]

Hi! We don't have conv3d yet. It is definitely something that an enterprising contributor could add, there is no requirement that it come from within Apple, and we are very encouraging of outside contributions.

That said I believe we would need to implement the kernel itself which is non-trivial. Though we have some Conv2D kernels as a good starting point.

[delneg]

3d ops are definitely needed, bumped into Conv3d missing while trying to substitute upsample_nearest3d from pytorch for MPS

Pytorch has a PR pending with Conv3d:

https://github.com/pytorch/pytorch/pull/114183/files

Maybe this will be helpful

[francescopisu]

@awni Is this something the community can do with a reasonable effort (by adding a new op in mlx/mlx/ops.py and making the required adjustments/extensions) or do we need help from within Apple ? Thank you for the nice framework!!