Add tests on all CFM classes

[kilianFatras]

What does this PR do?

This PR adds tests on all CFM classes. It adds a new tests folder and test_conditional_flow_matcher.py file.

First, I ensure that each main class function sample_location_and_conditional_flow runs properly (on different tensor sizes) and then I compare their outputs with my own computed outputs.

Before submitting

• Did you make sure title is self-explanatory and the description concisely explains the PR?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
• Did you test your PR locally with pytest command?
• Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Make sure you had fun coding 🙃

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (7cb209d) 34.23% compared to head (d700429) 35.63%.

Files	Patch %	Lines
torchcfm/conditional_flow_matching.py	85.71%	1 Missing ⚠️
torchcfm/optimal_transport.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   34.23%   35.63%   +1.39%     
==========================================
  Files          55       67      +12     
  Lines        6268     7417    +1149     
==========================================
+ Hits         2146     2643     +497     
- Misses       4122     4774     +652     

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[atong01]

Codecov check not passing but still happy as we have changed what is being checked. Looks good to merge by me.