The exact data.size() is (3, 15, 3, 256, 128), and the last batch is (2, 15, 3, 256, 128). The specific code is
arr = []
for data in data_loader:
    # data has shape (batch, num, channel, height, width)
    data = Variable(data.cuda())
    b, n, c, h, w = data.size()
    data = data.view(b*n, c, h, w)
    out = model(data) # out has shape (b*n, feat_dim)
    out = out.data.cpu()
    out = out.view(b, n, -1)
    out = out.mean(1) # so shape is (b, feat_dim)
    arr.append(out)
arr = torch.cat(arr, 0)

The origin is here 43.