[Feature] Refactor FedRunner and optimize trainer module

[rayrayraykk]

What's changed

1. Runner -> StandaloneRunner + DistributedRunner
2. Add README for Trainer module
3. Make num_val_batch, num_total_train_batch.... (related to cfg) as @Property method (Users could still use setattr or = to change these values, but the changes will make the @property function invalid.)

@property
def num_total_train_batch(self):
    if self.get('num_total_train_batch'):
        return self.get('num_total_train_batch')
    return self._calculate_batch_epoch_num(mode='train')[3]

5. Optimize doc string for hook functions and API ref.
6. Bind each metric with one and only key THE_LARGER_THE_BETTER
7. Move functions in utils to xxx.utils

Details

Coming soon.

[rayrayraykk]

Dicard this PR for the changes containing in #415 .