Add support for grouped convolutions

[arrufat]

In this PR I will try to add support for grouped convolutions in dlib.
I never had any interest/use for this kind of convolutions until yesterday, when I read this paper: A ConvNet for the 2020s.
The paper explores the main additions in Transformer networks and adds them to a convolutional network.
It makes use of recent additions to dlib:

• Add GELU activation layer #2204
• Add Layer Normalization #2213

Unfortunately, it makes also use of grouped convolutions, which are not currently supported in dlib.
That was the motivation I needed. So far I've written:

• forward gpu
• backward gpu
• forward cpu
• backward cpu

The gpu part is relatively easy, since it's just a matter of using the CuDNN API.
The cpu part might take longer to complete (I don't think I'll ever use it, but I will try to add it for completeness.)

I've already implemented the ConvNeXt models described in the paper, and the forward pass seems to work.
Let me know if the approach is sensible.

[pfeatherstone]

Yeah that paper is nice. I like these new simple building blocks. I hope they do actually work well beyond simple resnet modules.

[arrufat]

I got carried away with other stuff, I will finish this at some point, though 😅

[pfeatherstone]

Can this be implemented in the toeplitz matrix ?

[arrufat]

Can this be implemented in the toeplitz matrix ?

I was planning to get inspiration from here.
But feel free to step in, I don't think I will find time to do this soon, sadly enough :(

[rTreutlein]

I have a cpu implementation but I have to go through some company bureaucracy to be allowed to upload it.

[pfeatherstone]

Just noticed the toeplitz matrix isn't cached in the tensor_conv class. So there are a lot of allocations and deallocations. Right?

[davisking]

Just noticed the toeplitz matrix isn't cached in the tensor_conv class. So there are a lot of allocations and deallocations. Right?

There are, but there are other allocations and deallocations too. I doubt that one makes a meaningful difference to runtime speed considering what all goes on if someone is using this kind of model.

[pfeatherstone]

@davisking OK, I trust you. I had a quick look at the ncnn repo, and they have like a 100 specialisations of the convolution layer for different parameters like kernel sizes, groups, etc, whether or not the layer is 8-bit quantized, different architectures,... It looks like way too much work to do something similar in dlib to get CPU conv performance up to standard. Sorry this is unrelated to this PR. Just passing observation.

[davisking]

Yeah that's how you do conv on the CPU fast. The toeplitz matrix thing is a weird hack. I did it (as did others) because it's just a super easy way to support all the various conv types with all their different strides and all that. But fast conv code looks like this kind of stuff https://github.com/davisking/dlib/blob/master/dlib/image_transforms/spatial_filtering.h#L126. Or other similar kinds of setups. It depends on which kind of conv we are talking about.