Support groups in convolution

[seanmor5]

Grouped Convolutions are common enough that we should support them. Here's some discussion and papers on grouped convolutions: https://paperswithcode.com/method/grouped-convolution

XLA supports grouped convolutions with it's feature_group_count option (right now we always set it to 1). We will just need to add support for groups in the binary implementation of Nx.conv. This involves an additional iteration in the binary implementation as well as some additional shape checks.

XLA also supports batch_group_count for grouping parts of the batch. I see less value in adding this option as I haven't been able to find any applications that are all that common.