Backdoor example

Hi
I couldn't run the backdoor example
python federatedscope/main.py --cfg scripts/attack_exp_scripts/backdoor_attack/backdoor_badnet_fedavg_convnet2_on_femnist.yaml

An error has raised

**(fs) PS C:\temp\federatedscope> python federatedscope/main.py --cfg scripts/attack_exp_scripts/backdoor_attack/backdoor_badnet_fedavg_convnet2_on_femnist.yaml
Traceback (most recent call last):
File "C:\temp\federatedscope\federatedscope\main.py", line 28, in
init_cfg.merge_from_file(args.cfg_file)
File "c:\temp\federatedscope\federatedscope\core\configs\config.py", line 109, in merge_from_file
self.merge_from_other_cfg(cfg)
File "c:\temp\federatedscope\federatedscope\core\configs\config.py", line 124, in merge_from_other_cfg
_merge_a_into_b(cfg_other, self, self, [])
File "c:\temp\federatedscope\federatedscope\core\configs\yacs_config.py", line 526, in _merge_a_into_b
_merge_a_into_b(v, b[k], root, key_list + [k])
File "c:\temp\federatedscope\federatedscope\core\configs\yacs_config.py", line 543, in _merge_a_into_b
raise KeyError("Non-existent config key: {}".format(full_key))
KeyError: 'Non-existent config key: federate.local_update_steps'**

Thanks

Duaa

Reproduce the command with scripts/attack_exp_scripts/backdoor_attack/backdoor_badnet_fedavg_convnet2_on_femnist.yaml and inspect federatedscope/main.py, federatedscope/core/configs/config.py, and federatedscope/core/configs/yacs_config.py around the reported merge failure. Done means the backdoor example starts without the federate.local_update_steps KeyError.

Written by the indexing model from the issue text.