[Feature Request] Support Multi-task learning

[classicsong]

GraphStorm should support multi-task learning as its built-in capability.

• Unsupervised multi-task learning: Other than using link prediction as the sole supervision signal for unsupervised learning, one can add other supervision signals like reconstruct node features, predicting the motifs of specific nodes, etc (https://arxiv.org/abs/2007.10445).
• Entities with different node types or edges from different edge types of a graph can have different tasks associated with them. There are also cases when the same node types or edge types have multiple tasks associated with them.