support for mono channels

[faroit]

currently the model is expected to be running stereo. However, I also uploaded a mono model for speech enhancement where nb_channels = 1.

This can be implemented in the following way:

• if the model only required as a single channel audio spectrogram but the audio input signal has two channels, the input spectrogram can just be averaged across the channel dimension and after model(input) the output mask would need to be duplicated again.
• if the model only works with single channel and the audio input is also single channel, the preprocessing code would need to adjusted. This can be useful when we use the user input (from microphone) to record a mono signal and process it