I can’t find the documentation of lr_scheduler (only the method itself). Will someone please help me understand what exactly does these lines do and what (in general) does the lr_scheduler is inteneded to do?

The learning rate scheduler adjusts the learning rates stored in the optimizer.
In your example a step function is used to lower the learning rate after 7 steps with a multiplicative factor of gamma=0.1.
Assuming that your initial learning rate is 1e-1: