String parameter cannot be passed to `torchvision.ops.deform_conv2d`

🐛 Describe the bug

Bug Explanation

The current implementation of torchvision.ops.deform_conv2d implicitly assumes that padding is passed as a tuple or an integer, this means that if the padding is passed as either "same" or "valid" then torchvision is going to parse it as padding = ("same").

Sample Code

import torch

X = torch.randn((1,3,5,5))
torchvision.ops.deform_conv2d(
            X,
            torch.randn([1, 2 * 1 * 3 * 3, 3, 3]), #offset
            torch.randn([1, 3, 3, 3]),
            torch.randn([1]),
            stride=1,
            padding="zero",
            dilation=1,
            mask=None,
        )

Stacktrace

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
d:\Documents\Python Scripts\test.ipynb Cell 13 in <cell line: 2>()
      [1](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=0) X = torch.randn((1,3,5,5))
----> [2](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=1) torchvision.ops.deform_conv2d(
      [3](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=2)             X,
      [4](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=3)             torch.randn([1, 2 * 1 * 3 * 3, 3, 3]), #offset
      [5](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=4)             torch.randn([1, 3, 3, 3]),
      [6](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=5)             torch.randn([1]),
      [7](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=6)             stride=1,
      [8](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=7)             padding="zero",
      [9](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=8)             dilation=1,
     [10](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=9)             mask=None,
     [11](vscode-notebook-cell:/d%3A/Documents/Python%20Scripts/test.ipynb#X14sZmlsZQ%3D%3D?line=10)         )

File d:\Installations\Anaconda\lib\site-packages\torchvision\ops\deform_conv.py:77, in deform_conv2d(input, offset, weight, bias, stride, padding, dilation, mask)
     74     bias = torch.zeros(out_channels, device=input.device, dtype=input.dtype)
     76 stride_h, stride_w = _pair(stride)
---> 77 pad_h, pad_w = _pair(padding)
     78 dil_h, dil_w = _pair(dilation)
     79 weights_h, weights_w = weight.shape[-2:]

ValueError: too many values to unpack (expected 2)

Versions

Collecting environment information...
PyTorch version: 1.12.1
Is debug build: False
CUDA used to build PyTorch: 11.3
ROCM used to build PyTorch: N/A

OS: Microsoft Windows 10 Home
GCC version: (x86_64-posix-seh, Built by strawberryperl.com project) 8.3.0
Clang version: Could not collect
CMake version: version 3.24.0-rc1
Libc version: N/A

Python version: 3.8.8 (default, Apr 13 2021, 15:08:03) [MSC v.1916 64 bit (AMD64)] (64-bit runtime)
Python platform: Windows-10-10.0.19041-SP0
Is CUDA available: True
CUDA runtime version: 11.5.50
GPU models and configuration: GPU 0: NVIDIA GeForce GTX 1650
Nvidia driver version: 512.15
cuDNN version: Could not collect
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

Versions of relevant libraries:
[pip3] mypy-extensions==0.4.3
[pip3] numpy==1.23.1
[pip3] numpydoc==1.4.0
[pip3] torch==1.12.1
[pip3] torch-tb-profiler==0.4.0
[pip3] torchaudio==0.12.1
[pip3] torchvision==0.13.1
[conda] blas                      1.0                         mkl
[conda] cudatoolkit               11.3.1               h59b6b97_2
[conda] mkl                       2021.4.0           haa95532_640
[conda] mkl-service               2.4.0            py38h2bbff1b_0
[conda] mkl_fft                   1.3.1            py38h277e83a_0
[conda] mkl_random                1.2.2            py38hf11a4ad_0
[conda] numpy                     1.23.1           py38h7a0a035_0
[conda] numpy-base                1.23.1           py38hca35cd5_0
[conda] numpydoc                  1.4.0            py38haa95532_0
[conda] pytorch                   1.12.1          py3.8_cuda11.3_cudnn8_0    pytorch
[conda] pytorch-mutex             1.0                        cuda    pytorch
[conda] torch                     1.12.1                   pypi_0    pypi
[conda] torch-tb-profiler         0.4.0                    pypi_0    pypi
[conda] torchaudio                0.12.1               py38_cu113    pytorch
[conda] torchvision               0.13.1               py38_cu113    pytorch

Start in torchvision/ops/deform_conv.py at deform_conv2d, where the stack trace shows padding being passed to _pair. Check the existing operator tests for padding coverage and determine the intended handling for the reported string values. Done means the supported string-padding behavior is explicit and covered by tests without breaking integer or tuple padding.