[feat] Add F1 score workflow in validation step

[gudgud96]

All configs related to calculating F1 within validation step is under eval. Same set of config is used by test.py.

eval_first_n_examples: Only run F1 workflow for first N examples in validation set
eval_after_num_epoch: Only run F1 workflow after certain number of epochs
eval_per_epoch: Interval for running F1 workflow
exp_tag_name: Just insert some random name
audio_dir: Audio path for calculating F1 score
midi_dir: MIDI path for calculating F1 score

Also reduced num_workers for both train and validation to 4, as observed that 12 workers take long for process creation (especially after adding in F1 calculation workflow), but gives relatively minimal speedup.

** To use validation set for F1 calculation, please run midi_script.py to re-generate the MIDI files from stems for the validation set.

For training, refer to config/config_slakh_f1=0.65.yaml for the most up-to-date changes.