Add rougel for cross-device evaluation

[qbc2016]

To assess the performance of our fine-tuned model, we leverage the Rouge-L metric and conduct experiments with a large number of clients, utilizing the Dolly-15K dataset as our training corpus.
The Dolly-15K dataset encompasses a total of 15,015 data points, distributed across eight distinct tasks. For a more comprehensive evaluation, we allocate the final task exclusively for evaluation purposes, while dedicating the remaining ones to the training phase.
Our experimental setup involves a network of 200 clients, utilizing a Dirichlet distribution for data partitioning to emulate non-IID conditions across the client base.

To do the evaluation, run

python federatescope/eval/eval_for_rougel/eval.py --cfg federatescope/llm/baselime/xxx.yaml