[Fix] Make inputs & channels smaller in unittest

[MengzhangLI]

Make inputs and channels .py file of
(1) ./tests/test_models/test_backbones/,
(2) ./tests/test_models/test_necks/
and (3) ./tests/test_models/test_heads/ to avoid failure of CI on Github.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.82%. Comparing base (93f8b14) to head (8cc436a).
Report is 310 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1004      +/-   ##
==========================================
- Coverage   89.85%   89.82%   -0.04%     
==========================================
  Files         118      118              
  Lines        6558     6558              
  Branches     1019     1019              
==========================================
- Hits         5893     5891       -2     
- Misses        464      465       +1     
- Partials      201      202       +1     

Flag	Coverage Δ
unittests	89.82% <ø> (-0.04%)	⬇️

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