Refactor: Extract OUTPUT_DIR constant for visualization paths

[Copilot]

The output directory was hardcoded as "output" across five target classes in diffusion_sampling.py. This addresses feedback to centralize the configuration.

Changes

• Added module-level constant OUTPUT_DIR = "output" alongside TERMINAL_TIME_EPS
• Replaced hardcoded strings in visualize() methods for:
  • SimpleGaussianMixture
  • Grid25GaussianMixture
  • Posterior9of25GaussianMixture
  • Funnel
  • ManyWell

# Before
os.makedirs("output", exist_ok=True)
plt.savefig(f"output/{prefix}simple_gmm.png")

# After
os.makedirs(OUTPUT_DIR, exist_ok=True)
plt.savefig(f"{OUTPUT_DIR}/{prefix}simple_gmm.png")

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[josephdviviano]

lgtm

[codecov]

Codecov Report

❌ Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.33%. Comparing base (ad457f6) to head (b141f3f).
⚠️ Report is 3 commits behind head on relative_trajectory_balance.

Files with missing lines	Patch %	Lines
src/gfn/gym/diffusion_sampling.py	9.09%	10 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                      @@
##           relative_trajectory_balance     #459   +/-   ##
============================================================
  Coverage                        74.33%   74.33%           
============================================================
  Files                               48       48           
  Lines                             7111     7112    +1     
  Branches                           830      830           
============================================================
+ Hits                              5286     5287    +1     
  Misses                            1503     1503           
  Partials                           322      322           

Files with missing lines	Coverage Δ
src/gfn/gym/diffusion_sampling.py	48.75% <9.09%> (+0.12%)	⬆️

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