supporting voxelnext inference

[csinsgcc]

Motivation

supporting voxelnext inference

Modification

adding the module used in voxelnext：

• mmdet3d/configs/base/models/voxelnext.py
  
• mmdet3d/configs/voxelnext/
  
• mmdet3d/models/dense_heads/voxelnext_head.py
  
• mmdet3d/models/detectors/voxelnext.py
  
• mmdet3d/models/middle_encoders/sparse_encoder_voxelnext.py
  
• mmdet3d/models/task_modules/coders/voxelnext_bbox_coder.py
  

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[OpenMMLab-Assistant-004]

Hi @csinsgcc,

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[ZCMax]

SparseEncoderVoxelNext