Use torch.no_grad() decorator

[ZhiyuanChen]

No description provided.

[codecov]

Codecov Report

Merging #777 (c42c6d7) into master (0de9e14) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
- Coverage   63.22%   63.21%   -0.01%     
==========================================
  Files         143      143              
  Lines        8399     8398       -1     
  Branches     1502     1502              
==========================================
- Hits         5310     5309       -1     
  Misses       2820     2820              
  Partials      269      269              

Flag	Coverage Δ
unittests	63.21% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/runner/epoch_based_runner.py	78.84% <100.00%> (-0.21%)	⬇️

---

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 0de9e14...c42c6d7. Read the comment docs.