[Feature] Enhance client_cfg and add new dataset

[rayrayraykk]

1. Add new hetero dataset mini-graph-dc; how to use::

   python federatedscope/main.py --cfg federatedscope/gfl/baseline/mini_graph_dc/fedavg.yaml --client_cfg 
   federatedscope/gfl/baseline/mini_graph_dc/fedavg_per_client.yaml

2. Enable client_cfg in command line; how to use:

   python federatedscope/main.py --cfg federatedscope/gfl/baseline/mini_graph_dc/fedavg.yaml --client_cfg 
   federatedscope/gfl/baseline/mini_graph_dc/fedavg_per_client.yaml client_1.train.optimizer.lr 0.01

3. Enable client_cfg in HPO

[joneswong]

Looks good. It seems that client-specific configurations are fixed. TODO: the cartesian product of the client-wise search spaces to be explored by HPOptimizer.