feat: Added possibility to change the cache dir of datasets

[fg-mindee]

This PR adds the flexibility to override the cache folder used by VisionDataset, which makes it possible to have a flexible monkeypatch strategy for unittests.

cc @felixdittrich92

[charlesmindee]

Thanks for the PR!

[codecov]

Codecov Report

Merging #694 (943fa09) into main (74d9e65) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #694      +/-   ##
==========================================
- Coverage   96.16%   96.08%   -0.09%     
==========================================
  Files         124      124              
  Lines        4647     4648       +1     
==========================================
- Hits         4469     4466       -3     
- Misses        178      182       +4     

Flag	Coverage Δ
unittests	96.08% <100.00%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/datasets/base.py	95.45% <100.00%> (+0.10%)	⬆️
...dels/detection/differentiable_binarization/base.py	89.44% <0.00%> (-3.34%)	⬇️
doctr/models/recognition/predictor/pytorch.py	96.96% <0.00%> (+6.06%)	⬆️

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