Add custom hook by config file

[mzr1996]

Refer to #967
I noticed that mmcv hooks already have priority. Therefore, I assign different priorities to default hooks, so that, we can use priority value to indicate when a hook should run.
For example, I add a custom hook as:

@HOOKS.register_module()
class CustomHook(Hook):
    def __init__(self, p) -> None:
        super().__init__()
        self.p = p

    def __repr__(self) -> str:
        return f'<CustomHook> with priority {self.p}'

And edit the base_runner.call_hook function to show the effect.

    def call_hook(self, fn_name):
        """Call all hooks.

        Args:
            fn_name (str): The function name in each hook to be called, such as
                "before_train_epoch".
        """
        self.logger.info('-------------------- start call hook -------------------- ')
        for hook in self._hooks:
            self.logger.info(f"{hook} call {fn_name}")
            getattr(hook, fn_name)(self)
        self.logger.info('-------------------- end call hook -------------------- ')

Then, we can add custom hooks by config file with:

custom_train_hooks = [dict(type='CustomHook', priority=1, p=1),
                      dict(type='CustomHook', priority=89, p=89)]

And get the result (I use mmcls to show the effect):

2021-04-20 10:24:08,234 - mmcls - INFO - -------------------- start call hook -------------------- 
2021-04-20 10:24:08,234 - mmcls - INFO - <CustomHook> with priority 1 call before_run
2021-04-20 10:24:08,234 - mmcls - INFO - <mmcv.runner.hooks.lr_updater.StepLrUpdaterHook object at 0x7f80bcee1550> call before_run
2021-04-20 10:24:08,234 - mmcls - INFO - <mmcv.runner.hooks.optimizer.OptimizerHook object at 0x7f80bce80ee0> call before_run
2021-04-20 10:24:08,234 - mmcls - INFO - <mmcls.core.evaluation.eval_hooks.EvalHook object at 0x7f80bceaa8e0> call before_run
2021-04-20 10:24:08,234 - mmcls - INFO - <mmcv.runner.hooks.checkpoint.CheckpointHook object at 0x7f80bce99400> call before_run
2021-04-20 10:24:08,234 - mmcls - INFO - <mmcv.runner.hooks.iter_timer.IterTimerHook object at 0x7f80bce89ca0> call before_run
2021-04-20 10:24:08,235 - mmcls - INFO - <CustomHook> with priority 89 call before_run
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcv.runner.hooks.logger.text.TextLoggerHook object at 0x7f80bce89bb0> call before_run
2021-04-20 10:24:08,235 - mmcls - INFO - -------------------- end call hook -------------------- 
2021-04-20 10:24:08,235 - mmcls - INFO - -------------------- start call hook -------------------- 
2021-04-20 10:24:08,235 - mmcls - INFO - <CustomHook> with priority 1 call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcv.runner.hooks.lr_updater.StepLrUpdaterHook object at 0x7f80bcee1550> call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcv.runner.hooks.optimizer.OptimizerHook object at 0x7f80bce80ee0> call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcls.core.evaluation.eval_hooks.EvalHook object at 0x7f80bceaa8e0> call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcv.runner.hooks.checkpoint.CheckpointHook object at 0x7f80bce99400> call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcv.runner.hooks.iter_timer.IterTimerHook object at 0x7f80bce89ca0> call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <CustomHook> with priority 89 call before_train_epoch
2021-04-20 10:24:08,235 - mmcls - INFO - <mmcv.runner.hooks.logger.text.TextLoggerHook object at 0x7f80bce89bb0> call before_train_epoch
2021-04-20 10:24:08,236 - mmcls - INFO - -------------------- end call hook -------------------- 

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[codecov]

Codecov Report

Merging #970 (858ab1f) into master (f61295d) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #970      +/-   ##
==========================================
+ Coverage   64.57%   64.81%   +0.23%     
==========================================
  Files         152      152              
  Lines        9810     9827      +17     
  Branches     1781     1788       +7     
==========================================
+ Hits         6335     6369      +34     
+ Misses       3145     3129      -16     
+ Partials      330      329       -1     

Flag	Coverage Δ
unittests	64.81% <100.00%> (+0.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/runner/base_runner.py	77.41% <100.00%> (+6.83%)	⬆️
mmcv/runner/hooks/lr_updater.py	69.42% <0.00%> (+0.27%)	⬆️
mmcv/ops/deform_conv.py	61.76% <0.00%> (+0.28%)	⬆️
mmcv/runner/iter_based_runner.py	56.83% <0.00%> (+0.71%)	⬆️
mmcv/runner/hooks/logger/pavi.py	70.58% <0.00%> (+1.47%)	⬆️
mmcv/runner/priority.py	68.42% <0.00%> (+10.52%)	⬆️

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[ZwwWayne]

Overall the modification LGTM. Could you add a unit test to test those priorities of hooks with custom hooks, e.g., using the example you show in the PR message?

[mzr1996]

Overall the modification LGTM. Could you add a unit test to test those priorities of hooks with custom hooks, e.g., using the example you show in the PR message?

Ok, I have added the unittest.