refactor: Removed contiguous param since it's included in torch>=1.7

[fg-mindee]

This PR introduces the following modifications:

• removes the Pytorch to force params in a contiguous chunk to speed up optimization (now it's included in the core library)
• updates the CI & documentation accordingly
• removes unneeded loader operations in training scripts

Any feedback is welcome!

[codecov]

Codecov Report

Merging #756 (3f3ffce) into main (45e1647) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #756      +/-   ##
==========================================
- Coverage   96.20%   96.12%   -0.09%     
==========================================
  Files         129      129              
  Lines        4799     4799              
==========================================
- Hits         4617     4613       -4     
- Misses        182      186       +4     

Flag	Coverage Δ
unittests	96.12% <ø> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
doctr/models/recognition/predictor/tensorflow.py	91.17% <0.00%> (-5.89%)	⬇️
doctr/models/builder.py	97.36% <0.00%> (-1.76%)	⬇️

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[charlesmindee]

Thanks for the PR