[Feature] Support PoolFormer in MMSegmentation 2.0

[MengzhangLI]

Motivation

Support PoolFormer in MMSeg 1.x branch.

0.x PR: #1537

Modification

Fix mIoU results without ResizeToMultiple transforms in test pipeline of master branch, which are lower than their real results.
Related PR: #2190

Here are five re-implementation results using MMSegmentation dev-1.x branch:

	mIoU	mIoU*	MMSEG dev-1.x re-implementation 5 times mIoU*
PoolFormer-S12	36.68	37.07	36.95, 36.62, 36.37, 36.06, 36.25
PoolFormer-S24	40.12	40.36	39.45, 40.08, 40.36, 39.35, 39.82
PoolFormer-S36	41.61	41.81	41.16, 41.33, 41.08, 40.55, 40.76
PoolFormer-M36	41.95	42.35	41.61, 42.22, 42.05, 41.83, 42.78
PoolFormer-M48	42.43	42.76	42.32, 42.07, 42.36, 41.67, 42.19

mIoU\* means `ResizeToMultiple` transform is adopted in test pipeline.

[MeowZheng]

fix ut

[codecov]

Codecov Report

Base: 83.53% // Head: 83.52% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (7fb9bb4) compared to base (f3cd44b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2191      +/-   ##
===========================================
- Coverage    83.53%   83.52%   -0.01%     
===========================================
  Files          138      139       +1     
  Lines         7792     7898     +106     
  Branches      1167     1185      +18     
===========================================
+ Hits          6509     6597      +88     
- Misses        1106     1112       +6     
- Partials       177      189      +12     

Flag	Coverage Δ
unittests	83.52% <ø> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmseg/datasets/transforms/loading.py	85.60% <0.00%> (-14.40%)	⬇️
mmseg/utils/__init__.py	100.00% <0.00%> (ø)
mmseg/datasets/__init__.py	100.00% <0.00%> (ø)
mmseg/datasets/transforms/__init__.py	100.00% <0.00%> (ø)
mmseg/utils/io.py	80.95% <0.00%> (ø)
mmseg/datasets/transforms/transforms.py	97.70% <0.00%> (+0.76%)	⬆️
mmseg/models/backbones/timm_backbone.py	45.83% <0.00%> (+8.33%)	⬆️

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