empty tensor inference backward compatible

[dreamerlin]

No description provided.

[zhouzaida]

please fix the CI

[codecov]

Codecov Report

Merging #1131 (6ddfa29) into master (59ed0dd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1131   +/-   ##
=======================================
  Coverage   67.64%   67.64%           
=======================================
  Files         159      159           
  Lines       10295    10295           
  Branches     1858     1858           
=======================================
  Hits         6964     6964           
  Misses       2965     2965           
  Partials      366      366           

Flag	Coverage Δ
unittests	67.64% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmcv/cnn/bricks/wrappers.py	54.20% <100.00%> (ø)

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