[Feature] Add Runner and Loop (#86)

* [Feature] Add Runner and Loop

* refine docstring

* refine docstring

* refine __init__ of Runner

* add Runner.save_checkpoint

* add comment for data_batch

* rename validation_cfg to val_cfg

* update branch

* update branch

* discard wear reference

* set seed for torch in dataloader worker

* refine comments

* reorder methods

* import module in __init__.py

* add runner.load_checkpoint

* rename attributes

* fix indent

* fix indent

* add comment for LOOPS

* add example for methods

* add example for methods

* refine examples

* add Runner.resume and refactor register_hooks

* remove type hint of runner in CheckpointHook

* rename before_run to before_train

* remove collate

* add before_run after_run for loop.run

* dummy collate_fn

* add more unit tests

* move call_hook('before_run') to runner

* minor fix

* add comments for collate_fn

* fix type hint

* refactor logic to load or resume checkpoint

* fix typo

* add comments for IterBasedTrainLoop

* refactor unit tests

* fix unit tests

* refactor

* fix setup_env

* refine

* minor fix

* Merge branch 'main' of github.com:open-mmlab/mmengine into zzd/add-runner

* minor fix

* minor fix

* refine

* refactor unit tests

* fix unit tests

* add launcher property

* add logger, message_hub and writer unit tests

* add cur_dataloader attribute for runner

* fix IterBasedTrainLoop

* add torch.no_grad decorator

* add master_only decorator for save_checkpoint

* add before_test_epoch in TestLoop

* refactor load_or_resume method

* refine docstring

* fix typo

* build_dataloader can handle dataset object

* add dump_config for runner

* collate_fn should not a lambda function

* minor fix

* refine docstring

* fix unit tests

* improve comments

* refine unit tests

* rename collate_fn to pseduo_collate

* add 'TODO' tag