[Fix] add missing mean/std defaults, add missing weight init for sar

[felixT2K]

some minor fixes before the models will be trained

[codecov]

Codecov Report

Merging #1212 (e866eaa) into main (d217141) will decrease coverage by 0.03%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##             main    #1212      +/-   ##
==========================================
- Coverage   95.08%   95.05%   -0.03%     
==========================================
  Files         150      150              
  Lines        6444     6452       +8     
==========================================
+ Hits         6127     6133       +6     
- Misses        317      319       +2     

Flag	Coverage Δ
unittests	95.05% <77.77%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
...s/detection/differentiable_binarization/pytorch.py	97.77% <ø> (ø)
doctr/models/detection/linknet/pytorch.py	98.18% <ø> (ø)
doctr/models/detection/linknet/tensorflow.py	94.89% <ø> (ø)
doctr/models/recognition/sar/pytorch.py	97.29% <75.00%> (-1.28%)	⬇️
doctr/models/recognition/vitstr/tensorflow.py	97.56% <100.00%> (ø)

... and 1 file with indirect coverage changes