[Enhancement] Support overall dastaloader settings

[MeowZheng]

Motivation

open-mmlab/mmpretrain#752

Modification

1. Add overall dataloader cfg in api/train.py
2. Add overall dataloader cfg in tools/test.py

[codecov]

Codecov Report

Merging #117 (ee7a689) into master (31568b1) will decrease coverage by 0.77%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
- Coverage   69.56%   68.78%   -0.78%     
==========================================
  Files          92       92              
  Lines        5092     5135      +43     
  Branches      721      727       +6     
==========================================
- Hits         3542     3532      -10     
- Misses       1438     1489      +51     
- Partials      112      114       +2     

Flag	Coverage Δ
unittests	68.78% <0.00%> (-0.74%)	⬇️

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

Impacted Files	Coverage Δ
mmflow/apis/train.py	20.61% <0.00%> (-0.66%)	⬇️
mmflow/datasets/pipelines/transforms.py	77.50% <0.00%> (-3.67%)	⬇️
mmflow/datasets/utils/flow_io.py	32.77% <0.00%> (-2.30%)	⬇️
mmflow/datasets/utils/__init__.py	100.00% <0.00%> (ø)
mmflow/datasets/dataset_wrappers.py	43.75% <0.00%> (ø)
mmflow/datasets/pipelines/loading.py	71.09% <0.00%> (+1.09%)	⬆️

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