[Enhance] Add extra dataloader settings in configs.

[mzr1996]

Motivation

In some situation, we need to use different dataloader settings like samples_per_gpu in train/val/test dataloader.

Modification

Use train_dataloader, val_dataloader and test_dataloader settings in the data field to specify different arguments.

Use cases (Optional)

data = dict(
    samples_per_gpu=64,
    workers_per_gpu=4,
    train=dict(type='xxx', ...),
    val=dict(type='xxx', ...),
    test=dict(type='xxx', ...),
    # Use different batch size during inference.
    val_dataloader=dict(samples_per_gpu=8, workers_per_gpu=2),
    test_dataloader=dict(samples_per_gpu=8, workers_per_gpu=2),
)

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #752 (e539a63) into dev (995f291) will increase coverage by 1.56%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev     #752      +/-   ##
==========================================
+ Coverage   85.11%   86.67%   +1.56%     
==========================================
  Files         124      126       +2     
  Lines        7690     7857     +167     
  Branches     1328     1356      +28     
==========================================
+ Hits         6545     6810     +265     
+ Misses        951      846     -105     
- Partials      194      201       +7     

Flag	Coverage Δ
unittests	86.58% <0.00%> (+1.56%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/apis/train.py	18.42% <0.00%> (-0.76%)	⬇️
mmcls/datasets/voc.py	33.33% <0.00%> (-5.13%)	⬇️
mmcls/core/evaluation/eval_metrics.py	80.00% <0.00%> (-2.36%)	⬇️
mmcls/datasets/__init__.py	100.00% <0.00%> (ø)
mmcls/models/backbones/resnet.py	100.00% <0.00%> (ø)
mmcls/models/backbones/__init__.py	100.00% <0.00%> (ø)
mmcls/models/backbones/cspnet.py	92.81% <0.00%> (ø)
mmcls/datasets/custom.py	100.00% <0.00%> (ø)
mmcls/datasets/utils.py	40.42% <0.00%> (+2.12%)	⬆️
mmcls/datasets/base_dataset.py	98.97% <0.00%> (+7.67%)	⬆️
... and 5 more

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