[Feature] add reduction for neck

[zzc98]

Motivation

In some cases, it is necessary to reduce the dimension of extracted feature, so Reduction is implemented in necks.

Use cases

model = dict(
    type='ImageClassifier',
    backbone=dict(
        type='ResNet',
        depth=101,
        num_stages=4,
        out_indices=(3,),
        style='pytorch'
    ),
    neck=[
        dict(type='GeneralizedMeanPooling'),
        dict(type='LinearReduction',
             in_channels=2048,
             out_channels=512,
             act_cfg=dict(type='ReLU'),
             norm_cfg=dict(type='BN1d')
             )
    ],
    head=dict(
        type='LinearClsHead',
        num_classes=1000,
        in_channels=512,
        loss=dict(type='CrossEntropyLoss', loss_weight=1.0),
        topk=(1, 5),
    ),
)

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.

[Ezra-Yu]

please rebase the branch to fix the conflict.

[codecov]

Codecov Report

Base: 0.02% // Head: 91.39% // Increases project coverage by +91.36% 🎉

Coverage data is based on head (7eaf7ca) compared to base (b8b31e9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev-1.x     #978       +/-   ##
============================================
+ Coverage     0.02%   91.39%   +91.36%     
============================================
  Files          121      131       +10     
  Lines         8217     9804     +1587     
  Branches      1368     1543      +175     
============================================
+ Hits             2     8960     +8958     
+ Misses        8215      648     -7567     
- Partials         0      196      +196     

Flag	Coverage Δ
unittests	91.39% <ø> (+91.36%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/apis/inference.py	0.00% <0.00%> (ø)
mmcls/datasets/transforms/compose.py
mmcls/models/backbones/mobilevit.py	91.15% <0.00%> (ø)
mmcls/models/backbones/deit3.py	94.52% <0.00%> (ø)
mmcls/models/backbones/edgenext.py	95.20% <0.00%> (ø)
mmcls/models/backbones/swin_transformer_v2.py	89.47% <0.00%> (ø)
mmcls/models/backbones/mvit.py	92.46% <0.00%> (ø)
mmcls/models/necks/reduction.py	100.00% <0.00%> (ø)
mmcls/models/utils/layer_scale.py	86.66% <0.00%> (ø)
mmcls/models/heads/efficientformer_head.py	93.10% <0.00%> (ø)
... and 121 more

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[Ezra-Yu]

LGTM.