How does Pytorch Dataloader handle variable size data?

I have a dataset that looks like below. That is the first item is the user id followed by the set of items which is clicked by the user.

0   24104   27359   6684
0   24104   27359
1   16742   31529   31485
1   16742   31529
2   6579    19316   13091   7181    6579    19316   13091
2   6579    19316   13091   7181    6579    19316
2   6579    19316   13091   7181    6579    19316   13091   6579
2   6579    19316   13091   7181    6579
4   19577   21608
4   19577   21608
4   19577   21608   18373
5   3541    9529
5   3541    9529
6   6832    19218   14144
6   6832    19218
7   9751    23424   25067   12606   26245   23083   12606

I define a custom dataset to handle my click log data.

import torch.utils.data as data
class ClickLogDataset(data.Dataset):
    def __init__(self, data_path):
        self.data_path = data_path
        self.uids = []
        self.streams = []

        with open(self.data_path, 'r') as fdata:
            for row in fdata:
                row = row.strip('\n').split('\t')
                self.uids.append(int(row[0]))
                self.streams.append(list(map(int, row[1:])))

    def __len__(self):
        return len(self.uids)

    def __getitem__(self, idx):
        uid, stream = self.uids[idx], self.streams[idx]
        return uid, stream

Then I use a DataLoader to retrieve mini batches from the data for training.

from torch.utils.data.dataloader import DataLoader
clicklog_dataset = ClickLogDataset(data_path)
clicklog_data_loader = DataLoader(dataset=clicklog_dataset, batch_size=16)

for uid_batch, stream_batch in stream_data_loader:
    print(uid_batch)
    print(stream_batch)

The code above returns differently from what I expected, I want stream_batch to be a 2D tensor of type integer of length 16. However, what I get is a list of 1D tensor of length 16, and the list has only one element, like below. Why is that ?

#stream_batch
[tensor([24104, 24104, 16742, 16742,  6579,  6579,  6579,  6579, 19577, 19577,
        19577,  3541,  3541,  6832,  6832,  9751])]

Solution

This is the way I do it:

def collate_fn_padd(batch):
    '''
    Padds batch of variable length

    note: it converts things ToTensor manually here since the ToTensor transform
    assume it takes in images rather than arbitrary tensors.
    '''
    ## get sequence lengths
    lengths = torch.tensor([ t.shape[0] for t in batch ]).to(device)
    ## padd
    batch = [ torch.Tensor(t).to(device) for t in batch ]
    batch = torch.nn.utils.rnn.pad_sequence(batch)
    ## compute mask
    mask = (batch != 0).to(device)
    return batch, lengths, mask

then I pass that to the dataloader class as a collate_fn.

There seems to be a giant list of different posts in the pytorch forum. Let me link to all of them. They all have answers of their own and discussions. It doesn't seem to me that there is one "standard way to do it" but if there is from an authoritative reference please share.

It would be nice that the ideal answer mentions

efficiency, e.g. if to do the processing in GPU with torch in the collate function vs numpy

things of that sort.

List:

bucketing: - https://discuss.pytorch.org/t/tensorflow-esque-bucket-by-sequence-length/41284