[Feature] Add Google Landmark v2 retrieval dataset

[zzc98]

Motivation

Support Google Landmark v2 retrieval dataset in 2019.

Use cases

train_pipeline = [
    dict(type='LoadImageFromFile'),
    dict(type='Resize', scale=384),
    dict(type='RandomCrop', crop_size=336),
    dict(type='RandomFlip', prob=0.5, direction='horizontal'),
    dict(type='PackClsInputs'),
]

test_pipeline = [
    dict(type='LoadImageFromFile'),
    dict(type='Resize', scale=384),
    dict(type='CenterCrop', crop_size=336),
    dict(type='PackClsInputs')
]

val_pipeline = test_pipeline
data_root = 'data/google-landmark/'
ann_prefix = 'data/google-landmark/metadata'
batch = 64

train_dataloader = dict(
    batch_size=batch,
    num_workers=4,
    dataset=dict(
        type=dataset_type,
        data_root=data_root,
        mode='train',
        train_set='clean', # 'clean' or 'full'
        anno_prefix=ann_prefix,
        pipeline=train_pipeline),
    sampler=dict(type='DefaultSampler', shuffle=True),
    persistent_workers=True
)

test_dataloader = dict(
    batch_size=batch,
    num_workers=4,
    dataset=dict(
        type=dataset_type,
        data_root=data_root,
        anno_prefix=ann_prefix,
        mode='test',
        test_set='public', # 'public', 'private' or 'full'
        pipeline=test_pipeline),
    sampler=dict(type='DefaultSampler', shuffle=False),
    persistent_workers=True
)

val_dataloader = test_dataloader

gallery_dataloader = dict(
    batch_size=batch,
    num_workers=4,
    dataset=dict(
        type=dataset_type,
        data_root=data_root,
        mode='index',
        anno_prefix=ann_prefix,
        pipeline=val_pipeline
    ),
    sampler=dict(type='DefaultSampler', shuffle=False))

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

All modified and coverable lines are covered by tests ✅

Project coverage is 91.68%. Comparing base (b8b31e9) to head (9b85f73).
Report is 156 commits behind head on dev-1.x.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev-1.x    #1056       +/-   ##
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+ Coverage     0.02%   91.68%   +91.65%     
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  Files          121      128        +7     
  Lines         8217     9592     +1375     
  Branches      1368     1604      +236     
============================================
+ Hits             2     8794     +8792     
+ Misses        8215      618     -7597     
- Partials         0      180      +180     

Flag	Coverage Δ
unittests	91.68% <100.00%> (+91.65%)	⬆️

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