[BUG] dataloader only return the first element of the sequence

## 🐛🐛 Bug Report

### ⚗️ Current Behavior
When I build a dataset with a sequence of images (like frames), everything works fine. But if I tranfer it to a pytorch dataloader, the loader can only return the first element of the sequence.

**Input Code**
["Creating Datasets with Sequences" official colab example](https://colab.research.google.com/drive/1HdQNTJhnFGDtv_zlWJ70tq_4l7-ty7jh?usp=sharing)

Append following code to the end:
```python
print(ds[0]["frames"].shape)
print(ds[1]["frames"].shape)
dataloader = ds.pytorch()
data_iter = iter(dataloader)
print(next(data_iter)["frames"].shape)
print(next(data_iter)["frames"].shape)
```
It returns:
```python
(600, 1080, 1920, 3)
(1050, 1080, 1920, 3)
torch.Size([1, 1080, 1920, 3])
torch.Size([1, 1080, 1920, 3])
```
But I hope It returns something like:
```python
(600, 1080, 1920, 3)
(1050, 1080, 1920, 3)
torch.Size([1, 600, 1080, 1920, 3])
torch.Size([1, 1050, 1080, 1920, 3])
```