[FEAT] Additional metrics for evaluating forecasting (RMSE+ MSE)

[martinb-ai]

Adding in RMSE and MSE metrics for accuracy calculations when performing backtesting on models.

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In terminal:

SDK:

[martinb-ai]

@DidierRLopes ☝️

[jmaslek]

Any reason we can't add the others from darts.metrics?

[martinb-ai]

No reason, just figured I would add based on user input. I will add in more if you'd like.

[jmaslek]

Thats fair. Only reason I ask is because I was going to try using the sdk for a kaggle comp that used SMAPE as the scoring metric, but I noticed it wasn't an option, so I didnt use it. Having as many is both helpful and overkill.

[martinb-ai]

@jmaslek Added smape for you :)