[Refactor] Fix TnT compatibility and verbose warning.

[mzr1996]

Motivation

TnT does not support return tuple now. And the warning in extract_feat will show in every call.

Modification

As the title

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 #436 (9ba3c96) into master (a8f4f82) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   77.27%   77.27%           
=======================================
  Files         100      100           
  Lines        5356     5356           
  Branches      883      883           
=======================================
  Hits         4139     4139           
  Misses       1098     1098           
  Partials      119      119           

Flag	Coverage Δ
unittests	77.27% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmcls/models/backbones/tnt.py	99.08% <100.00%> (ø)
mmcls/models/classifiers/image.py	86.84% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a8f4f82...9ba3c96. Read the comment docs.

[Junjun2016]

Should add a period after default value in docstring.

[mzr1996]

Should add a period after default value in docstring.

Good, but maybe in another pull request.

[Ezra-Yu]

LGTM.