fix: 🐛 assume_straight_pages in predictor

[charlesmindee]

This PR fixes the issue #646.
The detection predictor should receive the assume_straight_boxes flag when instantiated.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #647 (6875aaf) into main (95a81a1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #647   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files         112      112           
  Lines        4349     4349           
=======================================
  Hits         4178     4178           
  Misses        171      171           

Flag	Coverage Δ
unittests	96.06% <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/zoo.py	100.00% <100.00%> (ø)
doctr/models/zoo.py	100.00% <100.00%> (ø)

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[fg-mindee]

Thanks for the fix!