Setting `num_training_steps` in `schedule_opts` is not good in function `initialize_optimizer`

This line:

https://github.com/deepset-ai/FARM/blob/a5bda34f0731a6f50c80309d86c79e4710dee052/farm/modeling/optimization.py#L144

is not nice when the API user sets an explicit `schedule_opts` dictionary. It overwrites values. IMO it should be deleted.
What do you think?