format_only=True, # Only format and save the results to coco json file
outfile_prefix='./work_dirs/coco_detection/test') # The prefix of output json files
```
## Training and testing config
MMEngine’s runner uses Loop to control the training, validation, and testing processes. Users can set the maximum training epochs and validation intervals with these fields.
```
train_cfg = dict(
type='EpochBasedTrainLoop', # The training loop type. Refer to https://github.com/open-mmlab/mmengine/blob/main/mmengine/runner/loops.py
max_epochs=12, # Maximum training epochs
val_interval=1) # Validation intervals. Run validation every epoch.
val_cfg = dict(type='ValLoop') # The validation loop type
test_cfg = dict(type='TestLoop') # The testing loop type
```
## Optimization config
optim_wrapper is the field to configure optimization-related settings. The optimizer wrapper not only provides the functions of the optimizer, but also supports functions such as gradient clipping, mixed precision training, etc. Find more in optimizer wrapper tutorial.
```
optim_wrapper = dict( # Optimizer wrapper config
type='OptimWrapper', # Optimizer wrapper type, switch to AmpOptimWrapper to enable mixed precision training.
optimizer=dict( # Optimizer config. Support all kinds of optimizers in PyTorch. Refer to https://pytorch.org/docs/stable/optim.html#algorithms
momentum=0.9, # Stochastic gradient descent with momentum
weight_decay=0.0001), # Weight decay of SGD
clip_grad=None, # Gradient clip option. Set None to disable gradient clip. Find usage in https://mmengine.readthedocs.io/en/latest/tutorials/optimizer.html
)
```
param_scheduler is a field that configures methods of adjusting optimization hyperparameters such as learning rate and momentum. Users can combine multiple schedulers to create a desired parameter adjustment strategy. Find more in parameter scheduler tutorial and parameter scheduler API documents
```
param_scheduler = [
# Linear learning rate warm-up scheduler
dict(
type='LinearLR', # Use linear policy to warmup learning rate
start_factor=0.001, # The ratio of the starting learning rate used for warmup
by_epoch=False, # The warmup learning rate is updated by iteration
begin=0, # Start from the first iteration
end=500), # End the warmup at the 500th iteration
# The main LRScheduler
dict(
type='MultiStepLR', # Use multi-step learning rate policy during training
by_epoch=True, # The learning rate is updated by epoch
begin=0, # Start from the first epoch
end=12, # End at the 12th epoch
milestones=[8, 11], # Epochs to decay the learning rate
gamma=0.1) # The learning rate decay ratio
]
```
## Hook config
Users can attach Hooks to training, validation, and testing loops to insert some operations during running. There are two different hook fields, one is `default_hooks` and the other is `custom_hooks`.
`default_hooks` is a dict of hook configs, and they are the hooks must be required at the runtime. They have default priority which should not be modified. If not set, runner will use the default values. To disable a default hook, users can set its config to `None`. Find more in HOOK.
```
default_hooks = dict(
timer=dict(type='IterTimerHook'), # Update the time spent during iteration into message hub
logger=dict(type='LoggerHook', interval=50), # Collect logs from different components of Runner and write them to terminal, JSON file, tensorboard and wandb .etc
param_scheduler=dict(type='ParamSchedulerHook'), # update some hyper-parameters of optimizer
checkpoint=dict(type='CheckpointHook', interval=1), # Save checkpoints periodically
sampler_seed=dict(type='DistSamplerSeedHook'), # Ensure distributed Sampler shuffle is active
visualization=dict(type='DetVisualizationHook')) # Detection Visualization Hook. Used to visualize validation and testing process prediction results
```
`custom_hooks` is a list of all other hook configs. Users can develop their own hooks and insert them in this field.
