[egs] Fix the issue regarding X-UMX when receiving a mono audio as the input

[r-sawata]

Addressed the following issue pointed out by Marius via Slack:

Marius Miron 12:01 AM
I think there is a bug in XUMX multidomain loss. whenever you do 2 * i : 2 * i + 2, you should actually do n_channel * i : n_channel * i + n_channel
otherwise it fails for monaural. what do you think?

Marius Miron 12:06 AM
and also (in multidomain) this must be _Spectrogram(spec_power=spec_power, mono=False), otherwise it mixes two sources in mono

[mpariente]

Thanks @r-sawata !

@JorisCos would you like to have a look at unit tests and docs build? :-)