Make own dataset

Hi, thanks for this great work.

I am trying to make my own dataset, but I am getting an error when I try to load them into the model. how can i fix this? My code below.
Thanks.

```python3
import numpy as np
import os
from pypianoroll import Multitrack

dir = './my_dataset'

for midi in os.listdir(dir):
midi_data = Multitrack(os.path.join(dir, midi))
tracks = [track.pianoroll for track in midi_data.tracks]
sample = np.stack(tracks, axis=-1)

print(sample.shape)
np.save('./dataset/new_train.npy',sample)
```