I am implementing the Faster RCNN paper and for the part of implementing the region proposal network, each training sample should have an image as the input and the label as a list of all possible bounding boxes.
However, passing a list of arrays(the bounding boxes) does not work actually.
What is the solution for this ?
I am currently using the custom Dataset class for creating the dataset and a dataloader for loading.
> # Creating the training dataset
class DetectionDataset(Dataset):
    
    def __init__(self, data_frame, img_size = 512, is_transform=False):
        self.data_frame = data_frame
        self.is_transform = is_transform
        self.mean = np.array([104.00699, 116.66877, 122.67892])
        self.n_classes = 21
        self.img_size = img_size if isinstance(img_size, tuple) else (img_size, img_size)
        
    def __len__(self):
        return self.data_frame.shape[0]
    
    def __getitem__(self, i):
        d = self.data_frame.ix[i]
        filename = os.path.join(new_images_dir, d['fname'])+".jpg"
        image = io.imread(filename)
        #this code only uses one array(bounding box) at a time
        label = np.array(d[['xmin', 'ymin', 'xmax', 'ymax']], np.int32)
        image= np.array(image, dtype=np.uint8)
        sample = {'image': image, 'lbl':label}
        if self.is_transform:
            sample = self.transform(sample)
        return sample
    
    def transform(self, sample):
        img = sample['image']
        lbl = sample['lbl']
        img = img[:, :, ::-1]
        img = img.astype(np.float64)
        img -= self.mean
        img = m.imresize(img, (self.img_size[0], self.img_size[1]))
        # Resize scales images from 0 to 255, thus we need
        # to divide by 255.0
        img = img.astype(float) / 255.0
        # NHWC -> NCWH
        img = img.transpose(2, 0, 1)
        
        img = torch.from_numpy(img).float()
        lbl = np.array(lbl)
        lbl = torch.from_numpy(lbl).long()
        
        sample  = {'image': img, 'lbl': lbl}
        return sample

The error -
RuntimeError: Traceback (most recent call last):
  File "/Users/navneetmkumar/anaconda/lib/python3.5/site-packages/torch/utils/data/dataloader.py", line 40, in _worker_loop
    samples = collate_fn([dataset[i] for i in batch_indices])
  File "/Users/navneetmkumar/anaconda/lib/python3.5/site-packages/torch/utils/data/dataloader.py", line 106, in default_collate
    return {key: default_collate([d[key] for d in batch]) for key in batch[0]}
  File "/Users/navneetmkumar/anaconda/lib/python3.5/site-packages/torch/utils/data/dataloader.py", line 106, in <dictcomp>
    return {key: default_collate([d[key] for d in batch]) for key in batch[0]}
  File "/Users/navneetmkumar/anaconda/lib/python3.5/site-packages/torch/utils/data/dataloader.py", line 91, in default_collate
    return torch.stack(batch, 0, out=out)
  File "/Users/navneetmkumar/anaconda/lib/python3.5/site-packages/torch/functional.py", line 66, in stack
    return torch.cat(inputs, dim, out=out)
RuntimeError: inconsistent tensor sizes at /Users/soumith/miniconda2/conda-bld/pytorch_1502000696751/work/torch/lib/TH/generic/THTensorMath.c:2709