[Fix] Revise high workers_per_gpus

[MeowZheng]

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Motivation

When workers of dataloader are too many, it might affect CPU efficiency and speed.

Modification

1. reviseworkers_per_gpu=8 to workers_per_gpu=4

[codecov]

Codecov Report

Merging #1506 (1fc34c1) into master (239049c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1506   +/-   ##
=======================================
  Coverage   90.30%   90.30%           
=======================================
  Files         140      140           
  Lines        8345     8345           
  Branches     1403     1403           
=======================================
  Hits         7536     7536           
  Misses        571      571           
  Partials      238      238           

Flag	Coverage Δ
unittests	90.30% <ø> (ø)

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