Add memory_efficient arg to densenet docs

[ekagra-ranjan]

This PR adds the arg memory_efficient to densenet docs and also mentions its publication.
Relevant: #1003

[codecov-io]

Codecov Report

Merging #1090 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1090      +/-   ##
==========================================
- Coverage   64.65%   64.58%   -0.08%     
==========================================
  Files          68       68              
  Lines        5410     5410              
  Branches      830      830              
==========================================
- Hits         3498     3494       -4     
- Misses       1662     1664       +2     
- Partials      250      252       +2

Impacted Files	Coverage Δ
torchvision/models/densenet.py	86.79% <ø> (ø)	⬆️
torchvision/transforms/transforms.py	80.9% <0%> (-0.6%)	⬇️
torchvision/transforms/functional.py	71.09% <0%> (-0.29%)	⬇️

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[fmassa]

Thanks!