set start_epoch/start_iter

[YasminZhang]

Describe the feature
As load_from only loads the checkpoint without the optimizer's status, I may want to start the training using a new scheduler while being able to set the start_epoch.

I found that in the previous configs, we can set

runner.start_epoch/iter=10

However, as runner has been deprecated, we don't have such

train_cfg.start_epoch/iter=10