[Bug] Audio channels bug

- [x] I didn't find a similar issue already open.
- [x] I read the documentation (README AND Wiki)
- [x] I have installed FFMpeg
- [x] My problem is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)

## Description
When use the command:
```python
python3 -m spleeter separate -p spleeter/resources/2stems.json -o output ../Audio/BeiJingWelcome.mp3
```
it will raise a ValueError:
```python
ValueError: Cannot feed value of shape (10978, 2, 7) for Tensor 'mix_stft:0', which has shape '(None, 2049, 2)'
```
The reason is that there is an error in the source code
```python
spleeter/spleeter/separator.py
```
line 264, change:
```python
stft = stft[:, :2]
```
into:
```python
stft = stft[:,:,:2]
```
then it works well.