[Enhancement] Add persistent_workers=True in val_dataloader

[MeowZheng]

Motivation

presistent_workers=True in val_dataloader will speed up evaluation during training and not affect iter training time.

[codecov]

Codecov Report

Merging #63 (0bb3632) into master (607e1ed) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   67.07%   67.07%           
=======================================
  Files          83       83           
  Lines        4726     4726           
  Branches      678      678           
=======================================
  Hits         3170     3170           
  Misses       1453     1453           
  Partials      103      103           

Flag	Coverage Δ
unittests	67.07% <ø> (ø)

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Impacted Files	Coverage Δ
mmflow/datasets/pipelines/transforms.py	78.66% <0.00%> (ø)

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