Implement task descriptor Module

[AndreaCossu]

It would be nice to have a plugin that allows to concatenate to each input pattern a task descriptor. The task descriptor may be a simple one-hot task vector or something more refined, as in the A-GEM paper (processed by a neural network).

The plugin should be structured in such a way to make it easy to define a protocol to build a new task descriptor.

[AndreaCossu]

@AntonioCarta suggested that it may be a model feature. This would require a similar process than the one used to build the MultitaskModule model.

[AntonioCarta]

(If I remember the paper correctly) It should be really easy. It's an Embedding layer that uses task labels to index the embeddings.