feat: add crnn_mobilenet small & large ckpts for torch backend

[charlesmindee]

This PR adds both crnn_mobilenet_v3_large and crnn_mobilenet_v3_small pytorch ckpts.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #516 (37ab749) into main (14b376e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files         109      109           
  Lines        4183     4183           
=======================================
  Hits         3990     3990           
  Misses        193      193           

Flag	Coverage Δ
unittests	95.38% <ø> (ø)

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Impacted Files	Coverage Δ
doctr/models/recognition/crnn/pytorch.py	100.00% <ø> (ø)

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[fg-mindee]

Cheers!