[Enhancement] Enhance in vfe_layers creation

[zehuichen123]

Move one code out of the for-loop. Mentioned in #383. A re-created PR due to CLA.

[codecov]

Codecov Report

Merging #389 (960447f) into master (dabf0a2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #389   +/-   ##
=======================================
  Coverage   49.71%   49.71%           
=======================================
  Files         181      181           
  Lines       12451    12451           
  Branches     1951     1951           
=======================================
  Hits         6190     6190           
  Misses       5871     5871           
  Partials      390      390           

Flag	Coverage Δ
unittests	49.71% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet3d/models/voxel_encoders/voxel_encoder.py	54.40% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dabf0a2...960447f. Read the comment docs.