[Feature] Support build batch sampler. (#173)

mmengine/runner/runner.py

@@ -891,6 +891,21 @@ class Runner:
             # if `sampler_cfg` is not a valid type
             sampler = sampler_cfg
 
+        # build batch sampler
+        batch_sampler_cfg = dataloader_cfg.pop('batch_sampler', None)
+        if batch_sampler_cfg is None:
+            batch_sampler = None
+        elif isinstance(batch_sampler_cfg, dict):
+            batch_sampler = DATA_SAMPLERS.build(
+                batch_sampler_cfg,
+                default_args=dict(
+                    sampler=sampler,
+                    batch_size=dataloader_cfg.pop('batch_size')))
+        else:
+            # fallback to raise error in dataloader
+            # if `batch_sampler_cfg` is not a valid type
+            batch_sampler = batch_sampler_cfg
+
         # build dataloader
         init_fn: Optional[partial]
         if self.seed is not None:
@@ -909,8 +924,8 @@ class Runner:
         # in model.
         data_loader = DataLoader(
             dataset=dataset,
-            sampler=sampler,
-            batch_sampler=None,
+            sampler=sampler if batch_sampler is None else None,
+            batch_sampler=batch_sampler,
             collate_fn=pseudo_collate,
             worker_init_fn=init_fn,
             **dataloader_cfg)