Add tests on lambda_t, low sigma values and guided functions

[kilianFatras]

What does this PR do?

I have added some tests on conditional flow matcher classes. Namely, I am now testing lambda_t, low sigma values and guided functions with return_noise.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e96c52c) 35.81% compared to head (3f08702) 35.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   35.81%   35.90%   +0.09%     
==========================================
  Files          67       67              
  Lines        7419     7419              
==========================================
+ Hits         2657     2664       +7     
+ Misses       4762     4755       -7     

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[kilianFatras]

LGTM