Development for fl-nlp-hetero-tasks

[cheneydon]

The implementation of FL-hetero-nlp-tasks, including:

• Some NLP datasets: AGNews, CNN/DM, IMDB, MSQG, NewsQA, SQuAD;
• Some metrics for NLP tasks: ROUGE, BLEU, METEOR;
• Label smoothing and warm-up learning rate schedule;
• New Model, Trainer, Aggregator, Server/Client for Assign-Then-Contrast framework (https://arxiv.org/abs/2212.05789) for FL-hetero-nlp-tasks;
• Configurations, scripts and unittest

[xieyxclack]

LGTM