FedAvgM strategy: Add Nesterov Accelerated Gradient (NAG)

[gubertoli]

Issue

Description

This PR improves the FedAvgM implementation FedAvgM introducing the Nesterov Accelerated Gradient (NAG), the same as reported in the original FedAvgM paper:

https://arxiv.org/pdf/1909.06335.pdf

Related issues/PRs

N/A

Proposal

Introduce the calculation of NAG as part of the global model parameters update.

Explanation

The improvement of the introduction of NAG is reported on the FedAvgM baseline:

https://github.com/adap/flower/tree/main/baselines/fedavgm#comparison-between-the-custom-fedavgm-and-fedavgm

Checklist

• [ ] Implement proposed change
• [ ] Write tests
• [ ] Update documentation
• [ ] Update the changelog entry below
• [ ] Make CI checks pass
• [ ] Ping maintainers on Slack (channel #contributions)

Changelog entry

Any other comments?

PR aligned with @jafermarq