Fixed RuntimeError on variable validation batch size

[Luke-Chesley]

Description

Changed dimension in concat_sequences to cat along dim = 0. This fixes bug mentioned in linked issue.

Checklist

• Linked issues (if existing)
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b3fcf86) 90.19% compared to head (67ed8e5) 90.11%.
Report is 8 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1511      +/-   ##
==========================================
- Coverage   90.19%   90.11%   -0.09%     
==========================================
  Files          30       30              
  Lines        4724     4724              
==========================================
- Hits         4261     4257       -4     
- Misses        463      467       +4     

Flag	Coverage Δ
cpu	90.11% <100.00%> (-0.09%)	⬇️
pytest	90.11% <100.00%> (-0.09%)	⬇️

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