refactor: Unified return_preds flags across all tasks

[fg-mindee]

This PR introduces the following modifications:

• ensures all prediction call flags are named the same way ("return_boxes" --> "return_preds")
• reflected changes on unittests, and demo

Any feedback is welcome!

[codecov]

Codecov Report

Merging #741 (fbda1ac) into main (808081f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #741   +/-   ##
=======================================
  Coverage   96.05%   96.05%           
=======================================
  Files         129      129           
  Lines        4794     4794           
=======================================
  Hits         4605     4605           
  Misses        189      189           

Flag	Coverage Δ
unittests	96.05% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
doctr/models/detection/predictor/pytorch.py	94.44% <ø> (ø)
doctr/models/detection/predictor/tensorflow.py	94.44% <ø> (ø)
doctr/datasets/loader.py	100.00% <100.00%> (ø)
...s/detection/differentiable_binarization/pytorch.py	98.37% <100.00%> (ø)
...etection/differentiable_binarization/tensorflow.py	97.36% <100.00%> (ø)
doctr/models/detection/linknet/pytorch.py	97.82% <100.00%> (ø)
doctr/models/detection/linknet/tensorflow.py	100.00% <100.00%> (ø)
doctr/models/_utils.py	95.60% <0.00%> (-3.30%)	⬇️
doctr/models/builder.py	97.36% <0.00%> (+0.87%)	⬆️
doctr/models/recognition/predictor/tensorflow.py	97.05% <0.00%> (+5.88%)	⬆️

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[charlesmindee]

Thanks!