Migrate thnn_conv_depthwise2d from THC to ATen (#62281)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62281
Closes gh-24646, Closes gh-24647
There is no `TensorIterator` equivalent to these kernels so this is just
migrating the existing kernels over to the ATen style.
I've benchmarked for contiguous tensors with this script:
```
import torch
shape = (10, 10, 100, 100)
x = torch.randn(*shape, device='cuda')
w = torch.randn((10, 1, 5, 5), device='cuda')
for _ in range(100):
torch.nn.functional.conv2d(x, w, groups=10)
```
and similarly for backwards. I see these as the same to within measurement error.
| | Master Forward (us) | This PR Forward (us) |
|------------------:|:-------------------:|:--------------------:|
| Forward | 133.5 | 133.6 |
| Backward (input) | 1,102 | 1,119 |
| Backward (weight) | 2,220 | 2,217 |
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D29943062
Pulled By: ngimel
fbshipit-source-id: fc5d16496eb733743face7c5a14e532d7b8ee26a
