train_dataloader = dict(
    batch_size=batch_size,
    num_workers=batch_size,
    persistent_workers=True,
    sampler=dict(type="DefaultSampler", shuffle=True),
    batch_sampler=dict(type="AspectRatioBatchSampler"),
    dataset=dict(type="ConcatDataset", datasets=train_datasets),
)

File "/.../miniconda3/envs/mmdet2/lib/python3.8/site-packages/mmengine/config/config.py", line 105, in __missing__
    raise KeyError(name)
KeyError: 'pipeline'

self.model.cfg["train_dataloader"]['dataset']
{'datasets': [{...}, {...}], 'type': 'ConcatDataset'}

self.model.cfg["train_dataloader"]['dataset']['datasets']
[{'ann_file': 'annotations/instances_train.json', 'backend_args': None, 'data_prefix': {...}, 'data_root': '/mmdet/data/...', 'filter_cfg': {...}, 'metainfo': {...}, 'pipeline': [...], 'type': 'CocoDataset'}, {'ann_file': 'annotations/instances_train.json', 'backend_args': None, 'data_prefix': {...}, 'data_root': '/mmdet/data/...', 'filter_cfg': {...}, 'metainfo': {...}, 'pipeline': [...], 'type': 'CocoDataset'}]