[Fix] Change priority of DistEvalIterHook to 'LOW'

[ckkelvinchan]

Reference: #488

[codecov]

Codecov Report

Merging #489 (9fb2ad4) into master (3d25860) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #489   +/-   ##
=======================================
  Coverage   80.53%   80.53%           
=======================================
  Files         190      190           
  Lines       10340    10340           
  Branches     1533     1533           
=======================================
  Hits         8327     8327           
  Misses       1783     1783           
  Partials      230      230           

Flag	Coverage Δ
unittests	80.51% <ø> (ø)

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Impacted Files	Coverage Δ
mmedit/apis/train.py	18.00% <ø> (ø)

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