Add warning in docs of Resize about different results for PIL and tensors

[NicolasHug]

This PR adds a warning in the docs indicating that PIL images and tensors may be transformed differently, even on the same data. This is a source of confusion according to the comments in #2950 and explicitly documenting this may help

[NicolasHug]

Rendered doc: https://485448-73328905-gh.circle-artifacts.com/0/docs/transforms.html#torchvision.transforms.Resize

[fmassa]

I would instead mention that the outputs might be different because PIL applies antialiasing while torch tensor don't use antialiasing, and that the difference is only visible for downsampling the image, and upsampling should lead to similar results.

Also, cc @vfdev-5 for the work on adding antialiasing support to interpolate.

[codecov]

Codecov Report

Merging #3615 (6928b0d) into master (7d955df) will decrease coverage by 0.20%.
The diff coverage is n/a.

❗ Current head 6928b0d differs from pull request most recent head 45b7b12. Consider uploading reports for the commit 45b7b12 to get more accurate results

@@            Coverage Diff             @@
##           master    #3615      +/-   ##
==========================================
- Coverage   79.69%   79.48%   -0.21%     
==========================================
  Files         105      105              
  Lines        9824     9822       -2     
  Branches     1583     1582       -1     
==========================================
- Hits         7829     7807      -22     
- Misses       1517     1527      +10     
- Partials      478      488      +10     

Impacted Files	Coverage Δ
torchvision/transforms/functional.py	81.77% <ø> (-0.09%)	⬇️
torchvision/transforms/transforms.py	84.30% <ø> (ø)
torchvision/utils.py	62.09% <0.00%> (-16.94%)	⬇️
torchvision/datasets/folder.py	78.57% <0.00%> (+1.02%)	⬆️

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[fmassa]

Thanks!