Allow users to use no scheduler and use a custom scheduling plugin

[plonerma]

This PR introduces a flag (attach_default_scheduler) in the methods fine_tune and train of the trainer in order to allow the user to

1. use no scheduler when training or finetuning, and
2. use a custom scheduler plugin (at the moment, a passed scheduling plugin would be added on top of the default scheduler),

while still using the other defaults of these functions.

[alanakbik]

Makes sense, thanks for adding this @plonerma!