Train single channel model using left or right channel

[faroit]

Currently using train.py --nb-channels 1 will apply a downmix in the spectral domain inside the model to feed in only single channel audio.

However, I can think about applications where we do not have access to a wiener filter and therefore apply the model to each channel individually. In that case the performance might be better when the model was trained on just the left or the right channel. This can be fixed since we use channel swap augmentation.

[faroit]

@aliutkus you agree?

[aliutkus]

Provided default is channel swap in that case, yes