feat: add learning rate scheduler in TF training scripts

[charlesmindee]

This PR adds a CosineDecay lr scheduler in both tensorflow training scripts.
Any feedback is welcome!

[codecov]

Codecov Report

Merging #360 (9a55806) into main (db1e034) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #360      +/-   ##
==========================================
- Coverage   96.11%   96.05%   -0.06%     
==========================================
  Files          83       83              
  Lines        3426     3426              
==========================================
- Hits         3293     3291       -2     
- Misses        133      135       +2     

Flag	Coverage Δ
unittests	96.05% <ø> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
doctr/models/core.py	94.11% <0.00%> (-0.85%)	⬇️
...dels/detection/differentiable_binarization/base.py	91.30% <0.00%> (-0.63%)	⬇️

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[fg-mindee]

Nice, thanks for this improvement!