Thanks David, collate_fn was a good direction .  I wrote a simple code that maybe someone here can re-use. I wanted to make something that pads a generic dim, and I don’t use an RNN of any type so PackedSequence was a bit of overkill for me. It’s simple, but it works for me.
def pad_tensor(vec, pad, dim):
    """
    args:
        vec - tensor to pad
        pad - the size to pad to
        dim - dimension to pad

    return:
        a new tensor padded to 'pad' in dimension 'dim'
    """
    pad_size = list(vec.shape)
    pad_size[dim] = pad - vec.size(dim)
    return torch.cat([vec, torch.zeros(*pad_size)], dim=dim)


class PadCollate:
    """
    a variant of callate_fn that pads according to the longest sequence in
    a batch of sequences
    """

    def __init__(self, dim=0):
        """
        args:
            dim - the dimension to be padded (dimension of time in sequences)
        """
        self.dim = dim

    def pad_collate(self, batch):
        """
        args:
            batch - list of (tensor, label)

        reutrn:
            xs - a tensor of all examples in 'batch' after padding
            ys - a LongTensor of all labels in batch
        """
        # find longest sequence
        max_len = max(map(lambda x: x[0].shape[self.dim], batch))
        # pad according to max_len
        batch = map(lambda (x, y):
                    (pad_tensor(x, pad=max_len, dim=self.dim), y), batch)
        # stack all
        xs = torch.stack(map(lambda x: x[0], batch), dim=0)
        ys = torch.LongTensor(map(lambda x: x[1], batch))
        return xs, ys

    def __call__(self, batch):
        return self.pad_collate(batch)



to be used with the data loader:
train_loader = DataLoader(ds, ..., collate_fn=PadCollate(dim=0))