[Refactor] Merge all evaluation metrics into one module

[SinaTabakhi]

Considering the presence of multiple modules in PyKale focused on evaluation metrics, it may be beneficial to merge them.

The following two modules should be merged into one, with only metrics.py remaining:
metrics.py
losses.py

Additionally, when PR #361 is merged into PyKale, the distance.py module should also be merged into metrics.py.

The implementation of pairwise metrics in scikit-learn can help in effectively refactoring these modules.
pairwise.py in scikit-learn

[github-actions[bot]]

This issue has been automatically marked as stale due to lack of activity. It will be closed if no further activity occurs. Thank you!

[github-actions[bot]]

This issue is closed due to lack of activity. Feel free to reopen it if you still have questions.

[haipinglu]

@SinaTabakhi It will be better to capture this in (the same or new) discussions.