Use is_floating_point() to check for float image

[datumbox]

Tiny refactoring to use directly is_floating_point() to check if the tensor is of floating type. Unclear whether there was a specific reason why we did the check this way and what is the preferred idiom.

This came up on another PR #3238, so it's worth clarifying.

[codecov]

Codecov Report

Merging #3239 (a414ed6) into master (d0063f3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3239   +/-   ##
=======================================
  Coverage   73.45%   73.45%           
=======================================
  Files         102      102           
  Lines        9329     9329           
  Branches     1494     1494           
=======================================
  Hits         6853     6853           
  Misses       2012     2012           
  Partials      464      464           

Impacted Files	Coverage Δ
torchvision/transforms/functional_tensor.py	79.04% <100.00%> (ø)

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[fmassa]

Thanks!