[Bug] Layer normalisation not implemented for transformers

[teethoe]

Branch

main branch (mmpretrain version)

Describe the bug

I was running inference with the ViTPose model using the code from the MMPose tutorial on a custom image.
I have only changed the pose config and checkpoint to the following:

pose_config = MMPOSE_ROOT / Path('configs/body_2d_keypoint/topdown_heatmap/coco/td-hm_ViTPose-huge_8xb64-210e_coco-256x192.py')
pose_checkpoint = 'https://download.openmmlab.com/mmpose/v1/pretrained_models/mae_pretrain_vit_huge_20230913.pth'

Environment

/home/ubuntu/miniconda3/envs/openmmlab/lib/python3.8/site-packages/mmcv/__init__.py:20: UserWarning: On January 1, 2023, MMCV will release v2.0.0, in which it will remove components related to the training process and add a data transformation module. In addition, it will rename the package names mmcv to mmcv-lite and mmcv-full to mmcv. See https://github.com/open-mmlab/mmcv/blob/master/docs/en/compatibility.md for more details.
  warnings.warn(
{'sys.platform': 'linux',
 'Python': '3.8.19 (default, Mar 20 2024, 19:58:24) [GCC 11.2.0]',
 'CUDA available': True,
 'MUSA available': False,
 'numpy_random_seed': 2147483648,
 'GPU 0': 'NVIDIA A2',
 'CUDA_HOME': '/usr/local/cuda',
 'NVCC': 'Cuda compilation tools, release 12.1, V12.1.66',
 'GCC': 'gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0',
 'PyTorch': '2.3.0',
 'TorchVision': '0.18.0',
 'OpenCV': '4.10.0',
 'MMEngine': '0.10.4',
 'MMCV': '1.7.2',
 'MMPreTrain': '1.2.0+4e28c70'}

Other information

1. No modifications to the code or config.
2. The default norm_cfg of the encoder layer for mmpretrain.models.backbones.vision_transformer and any other transformer model is 'LN'. Layer normalisation for transformers was never implemented in mmpretrain.models.utils.norm and 'LN' is not registered in the MODELS registry.