upgrade mmcv version

[xiliu8006]

modify mmcv version requirements

[codecov]

Codecov Report

Merging #376 (3053f3e) into master (28828ee) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 3053f3e differs from pull request most recent head b77cc88. Consider uploading reports for the commit b77cc88 to get more accurate results

@@           Coverage Diff           @@
##           master     #376   +/-   ##
=======================================
  Coverage   49.55%   49.55%           
=======================================
  Files         177      177           
  Lines       11834    11834           
  Branches     1844     1844           
=======================================
  Hits         5864     5864           
  Misses       5610     5610           
  Partials      360      360           

Flag	Coverage Δ
unittests	49.55% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmdet3d/__init__.py	77.27% <100.00%> (ø)

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