Add GNNInf model

[vgsatorras]

Description of changes: This pull request introduces the multivariate model from Multivariate Time Series Forecasting with Latent Graph Inference coded in Pytorch.

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Label: New feature

[lostella]

Make sure to install black==22.1.0 and to run

black src test

before committing your code, so that it takes care of some styling issues

[lostella]

There's also a few complaints from flake8, see the bottom of this log file

[vgsatorras]

Hi Lorenzo, thank you for the feedback.

I have made the proposed changes:

• Now GNNInfEstimator inherits from Estimator.
• The print has been replaced by the proposed logger
• The style now follows the formats from black and flake8.

Feel free to let me know if further modifications are required.

Best, Victor

[kashif]

@vgsatorras do you have some helpers for the PEMS and METR-LA datasets for gluonts? Thanks!