[FEAT] Add MLPMultivariate model

[elephaint]

This PR adds a multivariate version of the simple MLP into neuralforecast and demonstrates its application in the Multivariate_with_TSMixer example notebook. The purpose of this model is mainly for demonstration purposes, and for its simplicity.

• Added MLPMultivariate and AutoMLPMultivariate models to NeuralForecast
• Included tests in core class
• Included performance evaluation in example notebook.
• Added unit tests for loss functions to all multivariate models
• Added safeguard in BaseMultivariate for unsupported loss functions (basically all distribution loss functions)

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