[Improve] support gradcam

[cir7]

Motivation

[Improvement] support gradcam

[codecov]

Codecov Report

Base: 76.00% // Head: 76.86% // Increases project coverage by +0.85% 🎉

Coverage data is based on head (cb9c6a1) compared to base (040bd8b).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head cb9c6a1 differs from pull request most recent head 6913897. Consider uploading reports for the commit 6913897 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2049      +/-   ##
===========================================
+ Coverage    76.00%   76.86%   +0.85%     
===========================================
  Files          137      137              
  Lines         9924     9919       -5     
  Branches      1627     1627              
===========================================
+ Hits          7543     7624      +81     
+ Misses        2029     1944      -85     
+ Partials       352      351       -1     

Flag	Coverage Δ
unittests	76.86% <100.00%> (+0.85%)	⬆️

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

Impacted Files	Coverage Δ
mmaction/apis/inference.py	43.75% <ø> (ø)
mmaction/datasets/transforms/formatting.py	83.49% <ø> (ø)
mmaction/models/heads/base.py	80.00% <100.00%> (+2.09%)	⬆️
mmaction/models/recognizers/base.py	60.71% <100.00%> (-2.21%)	⬇️
mmaction/utils/gradcam_utils.py	97.72% <100.00%> (+88.53%)	⬆️
mmaction/models/heads/tpn_head.py	85.71% <0.00%> (+2.38%)	⬆️
mmaction/models/backbones/resnet2plus1d.py	100.00% <0.00%> (+4.00%)	⬆️
mmaction/models/recognizers/recognizer2d.py	59.61% <0.00%> (+9.61%)	⬆️

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[ly015]

What about moving this to 'visualization/grad_cam.py'?

[cir7]

I will reorganize visualzation tools structure in pr #2092