fix: Fixes inplace target modifications for AbstractDatasets

[fg-mindee]

This PR introduces the following modifications:

• updates _read_sample to return a deepcopy of the target to prevent inplace modifications
• adds a dedicated unittest (accessing several times a specific target will return the same value each time now, whether there are inplace transforms or not)

Closes #840

Any feedback is welcome!

[codecov]

Codecov Report

Merging #848 (8fa962f) into main (a2b31cc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #848   +/-   ##
=======================================
  Coverage   94.84%   94.84%           
=======================================
  Files         133      133           
  Lines        5195     5197    +2     
=======================================
+ Hits         4927     4929    +2     
  Misses        268      268           

Flag	Coverage Δ
unittests	94.84% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
doctr/datasets/datasets/pytorch.py	100.00% <100.00%> (ø)
doctr/datasets/datasets/tensorflow.py	100.00% <100.00%> (ø)
doctr/transforms/functional/base.py	95.65% <0.00%> (ø)

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[charlesmindee]

Thanks for spotting this !