Custom mapper for normalizing bounding box

## Instructions To Reproduce the Issue:

I am trying to code custom DatasetMapper as provided in the tutorial of detectron2. This mapper is responsible for normalizing the bounding box coordinate for my custom dataset. I am not sure where exactly detectron2 does normalization, if it does so. If you can point out this as well, it would be nice. Currently, I am writing my piece of code for this.

1. Full runnable code or full changes you made:
```
< dataset_dict = DatasetCatalog.get(cfg.DATASETS.TRAIN[0])
def mapper(dataset_dict):
dataset_dict = copy.deepcopy(dataset_dict)
image = cv2.imread(dataset_dict["file_name"], cv2.IMREAD_COLOR)
image = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)
height, width = image.shape[:2]
auginput = T.AugInput(image)
transform = T.Resize((800, 800))(auginput)
image = torch.from_numpy(auginput.image.transpose(2, 0, 1))
annotations = []
for annotation in dataset_dict["annotations"]:
bbox = annotation["bbox"]
bbox = [bbox[0] / width, bbox[1] / height, bbox[2] / width, bbox[3] / height]
bbox = [bbox[0], bbox[1], bbox[0] + bbox[2], bbox[1] + bbox[3]]
annotations.append({
"bbox": bbox,
"bbox_mode": BoxMode.XYXY_ABS,
"category_id": annotation["category_id"]
})
return {
"image": image,
"instances": utils.annotations_to_instances(annotations, image.shape[:2])
}
return build_detection_train_loader(cfg, mapper=mapper)>
```
2. __Full logs__ or other relevant observations:
```

launch(
File "/home/jovyan/.local/lib/python3.8/site-packages/detectron2/engine/launch.py", line 62, in launch
main_func(*args)
File "main.py", line 80, in main
return trainer.train()
File "/home/jovyan/thesis_s2577712/DeFRCN_voc_format/thesis-pascal_voc_format/defrcn/engine/defaults.py", line 387, in train
super().train(self.start_iter, self.max_iter)
File "/home/jovyan/.local/lib/python3.8/site-packages/detectron2/engine/train_loop.py", line 134, in train
self.run_step()
File "/home/jovyan/.local/lib/python3.8/site-packages/detectron2/engine/train_loop.py", line 222, in run_step
data = next(self._data_loader_iter)
File "/home/jovyan/.local/lib/python3.8/site-packages/detectron2/data/common.py", line 180, in __iter__
w, h = d["width"], d["height"]
KeyError: 'width'>
```
## Environment:

```
---------------------- ------------------------------------------------------------------------
sys.platform linux
Python 3.8.10 (default, Nov 14 2022, 12:59:47) [GCC 9.4.0]
numpy 1.23.1
detectron2 0.3 @/home/jovyan/.local/lib/python3.8/site-packages/detectron2
Compiler GCC 7.3
CUDA compiler CUDA 10.1
detectron2 arch flags 3.7, 5.0, 5.2, 6.0, 6.1, 7.0, 7.5
DETECTRON2_ENV_MODULE
PyTorch 1.6.0+cu101 @/home/jovyan/.local/lib/python3.8/site-packages/torch
PyTorch debug build False
GPU available True
GPU 0,1 Tesla T4 (arch=7.5)
CUDA_HOME /usr/local/cuda
Pillow 9.4.0
torchvision 0.7.0+cu101 @/home/jovyan/.local/lib/python3.8/site-packages/torchvision
torchvision arch flags 3.5, 5.0, 6.0, 7.0, 7.5
fvcore 0.1.5.post20221221
cv2 4.7.0
---------------------- ------------------------------------------------------------------------
PyTorch built with:
- GCC 7.3
- C++ Version: 201402
- Intel(R) Math Kernel Library Version 2019.0.5 Product Build 20190808 for Intel(R) 64 architecture applications
- Intel(R) MKL-DNN v1.5.0 (Git Hash e2ac1fac44c5078ca927cb9b90e1b3066a0b2ed0)
- OpenMP 201511 (a.k.a. OpenMP 4.5)
- NNPACK is enabled
- CPU capability usage: AVX2
- CUDA Runtime 10.1
- 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_70,code=sm_70;-gencode;arch=compute_75,code=sm_75
- CuDNN 7.6.3
- Magma 2.5.2
- Build settings: BLAS=MKL, BUILD_TYPE=Release, CXX_FLAGS= -Wno-deprecated -fvisibility-inlines-hidden -DUSE_PTHREADPOOL -fopenmp -DNDEBUG -DUSE_FBGEMM -DUSE_QNNPACK -DUSE_PYTORCH_QNNPACK -DUSE_XNNPACK -DUSE_VULKAN_WRAPPER -O2 -fPIC -Wno-narrowing -Wall -Wextra -Werror=return-type -Wno-missing-field-initializers -Wno-type-limits -Wno-array-bounds -Wno-unknown-pragmas -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wno-unused-function -Wno-unused-result -Wno-unused-local-typedefs -Wno-strict-overflow -Wno-strict-aliasing -Wno-error=deprecated-declarations -Wno-stringop-overflow -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 -Wno-stringop-overflow, PERF_WITH_AVX=1, PERF_WITH_AVX2=1, PERF_WITH_AVX512=1, USE_CUDA=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_STATIC_DISPATCH=OFF,
```

Start at detectron2/data/common.py line 180 and inspect the dictionary returned by the mapper passed to build_detection_train_loader. Compare this with the DatasetMapper and annotations_to_instances flow mentioned in the issue to determine where bounding-box transformations occur. Done means training proceeds past the loader without the width/height error and the bounding boxes use the intended coordinate representation.

Written by the indexing model from the issue text.