[Fix] Use len(result) as batch size

[wangruohui]

fix #985

[codecov]

Codecov Report

Merging #987 (2b92d48) into master (5a99f58) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 2b92d48 differs from pull request most recent head 6333c56. Consider uploading reports for the commit 6333c56 to get more accurate results

@@           Coverage Diff           @@
##           master     #987   +/-   ##
=======================================
  Coverage   65.27%   65.27%           
=======================================
  Files         151      151           
  Lines        9619     9619           
  Branches     1741     1741           
=======================================
  Hits         6279     6279           
  Misses       3017     3017           
  Partials      323      323           

Flag	Coverage Δ
unittests	65.27% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmcv/engine/test.py	26.88% <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 5a99f58...6333c56. Read the comment docs.

[zhouzaida]

@dreamerlin