update detection transformation to allow tensor input

[buoyancy99]

Though official document says the rcnns accepts a list of images. But it also makes sense to accept a batched tensor directly. The change eliminates bugs triggered when passing in tensor

[codecov-io]

Codecov Report

Merging #1111 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1111      +/-   ##
==========================================
- Coverage   64.85%   64.79%   -0.06%     
==========================================
  Files          68       68              
  Lines        5417     5417              
  Branches      834      835       +1     
==========================================
- Hits         3513     3510       -3     
- Misses       1647     1648       +1     
- Partials      257      259       +2

Impacted Files	Coverage Δ
torchvision/models/detection/transform.py	78.3% <100%> (ø)	⬆️
torchvision/transforms/transforms.py	80.35% <0%> (-0.59%)	⬇️

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[fmassa]

Thanks!