[Docs] Add runner tutorial (#1082)

* add runner doc

* revised according to comments and remove non existing links

* revise according to comments

docs/runner.md

 ## Runner
 
-The runner module aims to help users to start training with less code, while stays
-flexible and configurable.
+The runner class is designed to manage the training. It eases the training process with less code demanded from users while staying flexible and configurable. The main features are as listed:
 
-Documentation and examples are still on going.
+- Support `EpochBasedRunner` and `IterBasedRunner` for different scenarios. Implementing customized runners is also allowed to meet customized needs.
+- Support customized workflow to allow switching between different modes while training. Currently, supported modes are train and val.
+- Enable extensibility through various hooks, including hooks defined in MMCV and customized ones.
+
+## EpochBasedRunner
+
+As its name indicates, workflow in `EpochBasedRunner` should be set based on epochs. For example, [('train', 2), ('val', 1)] means running 2 epochs for training and 1 epoch for validation, iteratively. And each epoch may contain multiple iterations. Currently, MMDetection uses `EpochBasedRunner` by default.
+
+Let's take a look at its core logic:
+
+```python
+# the condition to stop training
+while curr_epoch < max_epochs:
+    # traverse the workflow.
+    # e.g. workflow = [('train', 2), ('val', 1)]
+    for i, flow in enumerate(workflow):
+        # mode(e.g. train) determines which function to run
+        mode, epochs = flow
+        # epoch_runner will be either self.train() or self.val()
+        epoch_runner = getattr(self, mode)
+        # execute the corresponding function
+        for _ in range(epochs):
+            epoch_runner(data_loaders[i], **kwargs)
+```
+
+Currently, we support 2 modes: train and val. Let's take a train function for example and have a look at its core logic:
+
+```python
+# Currently, epoch_runner could be either train or val
+def train(self, data_loader, **kwargs):
+    # traverse the dataset and get batch data for 1 epoch
+    for i, data_batch in enumerate(data_loader):
+        # it will execute all before_train_iter function in the hooks registered. You may want to watch out for the order.
+        self.call_hook('before_train_iter')
+        # set train_mode as False in val function
+        self.run_iter(data_batch, train_mode=True, **kwargs)
+        self.call_hook('after_train_iter')
+   self.call_hook('after_train_epoch')
+```
+
+## IterBasedRunner
+
+Different from `EpochBasedRunner`, workflow in `IterBasedRunner` should be set based on iterations. For example, [('train', 2), ('val', 1)] means running 2 iters for training and 1 iter for validation, iteratively. Currently, MMSegmentation uses `IterBasedRunner` by default.
+
+Let's take a look at its core logic:
+
+```python
+# Although we set workflow by iters here, we might also need info on the epochs in some using cases. That can be provided by IterLoader.
+iter_loaders = [IterLoader(x) for x in data_loaders]
+# the condition to stop training
+while curr_iter < max_iters:
+    # traverse the workflow.
+    # e.g. workflow = [('train', 2), ('val', 1)]
+    for i, flow in enumerate(workflow):
+        # mode(e.g. train) determines which function to run
+        mode, iters = flow
+        # epoch_runner will be either self.train() or self.val()
+        iter_runner = getattr(self, mode)
+        # execute the corresponding function
+        for _ in range(iters):
+            iter_runner(iter_loaders[i], **kwargs)
+```
+
+Currently, we support 2 modes: train and val. Let's take a val function for example and have a look at its core logic:
+
+```python
+# Currently, iter_runner could be either train or val
+def val(self, data_loader, **kwargs):
+    # get batch data for 1 iter
+    data_batch = next(data_loader)
+    # it will execute all before_val_iter function in the hooks registered. You may want to watch out for the order.
+    self.call_hook('before_val_iter')
+    outputs = self.model.val_step(data_batch, self.optimizer, **kwargs)
+    self.outputs = outputs
+    self.call_hook('after_val_iter')
+```
+
+Other than the basic functionalities explained above, `EpochBasedRunner` and `IterBasedRunner` provide methods such as `resume`, `save_checkpoint` and `register_hook`. In case you are not familiar with the term Hook mentioned earlier, we will also provide a tutorial about it.(coming soon...) Essentially, a hook is functionality to alter or augment the code behaviors through predefined api. It allows users to have their own code called under certain circumstances. It makes code extensible in a non-intrusive manner.
+
+## A Simple Example
+
+We will walk you through the usage of runner with a classification task. The following code only contains essential steps for demonstration purposes. The following steps are necessary for any training tasks.
+
+**(1) Initialize dataloader, model, optimizer, etc.**
+
+```python
+# initialize model
+model=...
+# initialize optimizer, typically, we set: cfg.optimizer = dict(type='SGD', lr=0.1, momentum=0.9, weight_decay=0.0001)
+optimizer = build_optimizer(model, cfg.optimizer)
+# intialize the dataloader corresponding to the workflow(train/val)
+data_loaders = [
+        build_dataloader(
+            ds,
+            cfg.data.samples_per_gpu,
+            cfg.data.workers_per_gpu,
+            ...) for ds in dataset
+    ]
+```
+
+**(2) Initialize runner**
+
+```python
+runner = build_runner(
+    # cfg.runner is typically set as:
+    # runner = dict(type='EpochBasedRunner', max_epochs=200)
+    cfg.runner,
+    default_args=dict(
+        model=model,
+        batch_processor=None,
+        optimizer=optimizer,
+        logger=logger))
+```
+
+**(3) Register training hooks and customized hooks.**
+
+```python
+# register defalt hooks neccesary for traning
+runner.register_training_hooks(
+    # configs of learning rate，it is typically set as:
+    # lr_config = dict(policy='step', step=[100, 150])
+    cfg.lr_config,
+    # configuration of optimizer, e.g. grad_clip
+    optimizer_config,
+    # configuration of saving checkpoints, it is typically set as:
+    # checkpoint_config = dict(interval=1)，saving checkpoints every epochs
+    cfg.checkpoint_config,
+    # configuration of logs
+    cfg.log_config,
+    ...)
+
+# register customized hooks
+# say we want to enable ema, then we could set custom_hooks=[dict(type='EMAHook')]
+if cfg.get('custom_hooks', None):
+    custom_hooks = cfg.custom_hooks
+    for hook_cfg in cfg.custom_hooks:
+        hook_cfg = hook_cfg.copy()
+        priority = hook_cfg.pop('priority', 'NORMAL')
+        hook = build_from_cfg(hook_cfg, HOOKS)
+        runner.register_hook(hook, priority=priority)
+```
+
+Then, we can use `resume` or `load_checkpoint` to load existing weights.
+
+**(4) Start training**
+
+```python
+# workflow is typically set as: workflow = [('train', 1)]
+# here the training begins.
+runner.run(data_loaders, cfg.workflow)
+```
+
+Let's take `EpochBasedRunner` for example and go a little bit into details about setting workflow:
+
+- Say we only want to put train in the workflow, then we can set: workflow = [('train', 1)]. The runner will only execute train iteratively in this case.
+- Say we want to put both train and val in the workflow, then we can set: workflow = [('train', 3), ('val',1)]. The runner will first execute train for 3 epochs and then switch to val mode and execute val for 1 epoch. The workflow will be repeated until the current epoch hit the max_epochs.
+- Workflow is highly flexible. Therefore, you can set workflow = [('val', 1), ('train',1)] if you would like the runner to validate first and train after.
+
+The code we demonstrated above is already in `train.py` in MM repositories. Simply modify the corresponding keys in the configuration files and the script will execute the expected workflow automatically.