[Evaluation] Add metrics for evaluating regression tasks

[bcebere]

Feature Description

One of the major tasks of the library is evaluating the quality of the models and evaluating the AutoML objectives.

To that end, metrics are needed for every supported problem type.

One of them is evaluating regression tasks. The library should offer an API for using any of these metrics, testing the predicted values against the ground truth.

Important metrics to cover here:

• r2" R^2(coefficient of determination) regression score function.
• mse: Mean squared error regression loss.
• mae: Mean absolute error regression loss.

AP reference: https://github.com/vanderschaarlab/autoprognosis/blob/main/src/autoprognosis/utils/tester.py