`build_resnet_backbone` has wrong `size_divisibility`

## Instructions To Reproduce the 🐛 Bug:
1. Full runnable code or full changes you made:
```
build_backbone(cfg) when BACKBONE.NAME: 'build_resnet_backbone'
```

## Expected behavior:
```
self.backbone.size_divisibility should be `32` when RESNETS.OUT_FEATURES: [ 'res3', 'res4', 'res5' ]
```
But it outputs `0`

## Environment:
```
------------------------------- ----------------------------------------------------------------------------
sys.platform linux
Python 3.10.10 (main, Mar 21 2023, 18:45:11) [GCC 11.2.0]
numpy 1.23.5
detectron2 0.6 @/software/anaconda3/envs/rh/lib/python3.10/site-packages/detectron2
Compiler GCC 9.4
CUDA compiler CUDA 11.8
detectron2 arch flags 8.6
DETECTRON2_ENV_MODULE
PyTorch 2.0.0 @/software/anaconda3/envs/rh/lib/python3.10/site-packages/torch
PyTorch debug build False
torch._C._GLIBCXX_USE_CXX11_ABI False
GPU available Yes
GPU 0,1 NVIDIA GeForce RTX 3090 (arch=8.6)
Driver version 520.61.05
CUDA_HOME /usr/local/cuda
Pillow 9.4.0
torchvision 0.15.0 @/software/anaconda3/envs/rh/lib/python3.10/site-packages/torchvision
torchvision arch flags 3.5, 5.0, 6.0, 7.0, 7.5, 8.0, 8.6
fvcore 0.1.5.post20221221
iopath 0.1.9
cv2 4.7.0
------------------------------- ----------------------------------------------------------------------------
PyTorch built with:
- GCC 9.3
- C++ Version: 201703
- Intel(R) oneAPI Math Kernel Library Version 2021.4-Product Build 20210904 for Intel(R) 64 architecture applications
- Intel(R) MKL-DNN v2.7.3 (Git Hash 6dbeffbae1f23cbbeae17adb7b5b13f1f37c080e)
- OpenMP 201511 (a.k.a. OpenMP 4.5)
- LAPACK is enabled (usually provided by MKL)
- NNPACK is enabled
- CPU capability usage: AVX2
- CUDA Runtime 11.8
- NVCC architecture flags: -gencode;arch=compute_37,code=sm_37;-gencode;arch=compute_50,code=sm_50;-gencode;arch=compute_60,code=sm_60;-gencode;arch=compute_61,code=sm_61;-gencode;arch=compute_70,code=sm_70;-gencode;arch=compute_75,code=sm_75;-gencode;arch=compute_80,code=sm_80;-gencode;arch=compute_86,code=sm_86;-gencode;arch=compute_90,code=sm_90;-gencode;arch=compute_37,code=compute_37
- CuDNN 8.7
- Magma 2.6.1
- Build settings: BLAS_INFO=mkl, BUILD_TYPE=Release, CUDA_VERSION=11.8, CUDNN_VERSION=8.7.0, CXX_COMPILER=/opt/rh/devtoolset-9/root/usr/bin/c++, CXX_FLAGS= -D_GLIBCXX_USE_CXX11_ABI=0 -fabi-version=11 -Wno-deprecated -fvisibility-inlines-hidden -DUSE_PTHREADPOOL -DNDEBUG -DUSE_KINETO -DLIBKINETO_NOROCTRACER -DUSE_FBGEMM -DUSE_QNNPACK -DUSE_PYTORCH_QNNPACK -DUSE_XNNPACK -DSYMBOLICATE_MOBILE_DEBUG_HANDLE -O2 -fPIC -Wall -Wextra -Werror=return-type -Werror=non-virtual-dtor -Werror=bool-operation -Wnarrowing -Wno-missing-field-initializers -Wno-type-limits -Wno-array-bounds -Wno-unknown-pragmas -Wunused-local-typedefs -Wno-unused-parameter -Wno-unused-function -Wno-unused-result -Wno-strict-overflow -Wno-strict-aliasing -Wno-error=deprecated-declarations -Wno-stringop-overflow -Wno-psabi -Wno-error=pedantic -Wno-error=redundant-decls -Wno-error=old-style-cast -fdiagnostics-color=always -faligned-new -Wno-unused-but-set-variable -Wno-maybe-uninitialized -fno-math-errno -fno-trapping-math -Werror=format -Werror=cast-function-type -Wno-stringop-overflow, LAPACK_INFO=mkl, PERF_WITH_AVX=1, PERF_WITH_AVX2=1, PERF_WITH_AVX512=1, TORCH_DISABLE_GPU_ASSERTS=ON, TORCH_VERSION=2.0.0, USE_CUDA=ON, USE_CUDNN=ON, USE_EXCEPTION_PTR=1, USE_GFLAGS=OFF, USE_GLOG=OFF, USE_MKL=ON, USE_MKLDNN=ON, USE_MPI=OFF, USE_NCCL=ON, USE_NNPACK=ON, USE_OPENMP=ON, USE_ROCM=OFF,

Testing NCCL connectivity ... this should not hang.
NCCL succeeded.
```

The issue identifies build_backbone(cfg), build_resnet_backbone, and RESNETS.OUT_FEATURES; start by tracing that entry point and the backbone construction logic. Verify how size_divisibility is determined for res3, res4, and res5, and consider the work complete when the resulting backbone reports 32 for that configuration.

Written by the indexing model from the issue text.