fix: Fixed activation duplication in conv_sequence

[fg-mindee]

This PR introduces the following changes:

• fixed the duplication of activation in conv_sequece in a specific case
• added unittest to check the fix

This is a breaking change as previous checkpoints won't be working anymore.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #101 (f78563c) into main (d13b8ce) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   97.82%   97.94%   +0.12%     
==========================================
  Files          26       26              
  Lines         826      826              
==========================================
+ Hits          808      809       +1     
+ Misses         18       17       -1     

Flag	Coverage Δ
unittests	97.94% <100.00%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
doctr/models/utils.py	100.00% <100.00%> (+2.22%)	⬆️

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