fix: 🐛 bug canvas dtype for threshold target

[charlesmindee]

The threshold target map (of the DB) was set to be an uint8 tensor, but it should contains values between 0 and 1 (which were cast to 0).
This is now fixed.

[codecov]

Codecov Report

Merging #645 (8d8fa33) into main (95a81a1) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
- Coverage   96.06%   96.04%   -0.03%     
==========================================
  Files         112      112              
  Lines        4349     4349              
==========================================
- Hits         4178     4177       -1     
- Misses        171      172       +1     

Flag	Coverage Δ
unittests	96.04% <100.00%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	91.30% <100.00%> (-0.63%)	⬇️

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[fg-mindee]

Thanks a lot!