[Feature]Add loader args to config

[VVsssssk]

Motivation

Such a design only allows setting the different batch sizes in Val and test data loader.
Adding train_dataloader, val_dataloader, and test_dataloader in the data field of config allows more flexible usages.
A new example looks like below:

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

This PR refers to MMDet PR #7668.

BC-breaking (Optional)

None

[codecov]

Codecov Report

Merging #1388 (63211c0) into master (c20b6cb) will increase coverage by 0.10%.
The diff coverage is 79.16%.

❗ Current head 63211c0 differs from pull request most recent head e068288. Consider uploading reports for the commit e068288 to get more accurate results

@@            Coverage Diff             @@
##           master    #1388      +/-   ##
==========================================
+ Coverage   51.20%   51.31%   +0.10%     
==========================================
  Files         212      213       +1     
  Lines       18280    18350      +70     
  Branches     2978     2996      +18     
==========================================
+ Hits         9361     9416      +55     
- Misses       8362     8371       +9     
- Partials      557      563       +6     

Flag	Coverage Δ
unittests	51.31% <79.16%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/utils/setup_env.py	68.00% <60.00%> (-4.73%)	⬇️
mmdet3d/utils/compat_cfg.py	80.30% <80.30%> (ø)
mmdet3d/utils/__init__.py	100.00% <100.00%> (ø)

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