[Feature] Add vis-cam tool

[Ezra-Yu]

Motivation

Add tool to visualize cam

Modification

Please briefly describe what modification is made in this PR.

Use cases

python tools/visualizations/vis_cam.py demo/bird.JPEG configs/resnet/resnet50_8xb32_in1k.py \
    https://download.openmmlab.com/mmclassification/v0/resnet/resnet50_batch256_imagenet_20200708-cfb998bf.pth \
    --target-layers model.backbone.layer4.2

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 #577 (9d5c053) into master (643fb19) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
+ Coverage   81.31%   81.34%   +0.02%     
==========================================
  Files         115      116       +1     
  Lines        6654     6722      +68     
  Branches     1142     1156      +14     
==========================================
+ Hits         5411     5468      +57     
- Misses       1095     1103       +8     
- Partials      148      151       +3     

Flag	Coverage Δ
unittests	81.34% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/backbones/swin_transformer.py	91.90% <100.00%> (+0.04%)	⬆️
mmcls/models/backbones/t2t_vit.py	94.44% <100.00%> (+0.04%)	⬆️
mmcls/datasets/__init__.py	100.00% <0.00%> (ø)
mmcls/datasets/samplers/__init__.py	100.00% <0.00%> (ø)
mmcls/datasets/samplers/repeat_aug.py	85.10% <0.00%> (ø)
mmcls/datasets/builder.py	46.15% <0.00%> (+3.60%)	⬆️
mmcls/apis/train.py	27.77% <0.00%> (+6.25%)	⬆️
mmcls/datasets/samplers/distributed_sampler.py	28.00% <0.00%> (+6.26%)	⬆️

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[mzr1996]

LGTM