Garbage Collecting past node contributions

[JulienVig]

At every round, nodes receive weight updates from peers or the server which they store and/or aggregate to update their local model. Currently, weights from past rounds are kept indefinitely and accumulates over time. This is not prohibitive but is not efficient.

Specifically, in the federated case, the server results and the aggregated results are stored indefinitely

// client/federated.ts
const serverResult = await this.sendPayloadAndReceiveResult() // creates new tensors

// aggregator/mean.ts
this.aggregate() // creates new tensors