upgrade to build_anchor_generator

[mickeyouyou]

miniconda3/envs/open-mmlab/lib/python3.7/site-packages/mmdet/core/anchor/builder.py:17: UserWarning: build_anchor_generator would be deprecated soon, please use build_prior_generator

Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #941 (7405d6a) into master (21e4b12) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #941   +/-   ##
=======================================
  Coverage   49.33%   49.33%           
=======================================
  Files         210      210           
  Lines       16021    16021           
  Branches     2556     2556           
=======================================
  Hits         7904     7904           
  Misses       7614     7614           
  Partials      503      503           

Flag	Coverage Δ
unittests	49.33% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmdet3d/core/anchor/__init__.py	100.00% <100.00%> (ø)
mmdet3d/models/dense_heads/anchor3d_head.py	29.84% <100.00%> (ø)
mmdet3d/models/detectors/imvoxelnet.py	50.00% <100.00%> (ø)

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