Add seq start position when applying RoPE encoding

[laggui]

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Checklist

• Confirmed that run-checks all script has been executed.

Changes

Added apply(x, start) method to compute the rotary positional encoding with a start position, which is required for autoregressive LLMs (e.g., Llama) where the encoding is not always applied from the start of the sequence.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.41%. Comparing base (8c01444) to head (aa42cef).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1796   +/-   ##
=======================================
  Coverage   86.41%   86.41%           
=======================================
  Files         737      737           
  Lines       86009    86016    +7     
=======================================
+ Hits        74326    74333    +7     
  Misses      11683    11683           

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