take Geneformer out of CI

[mlin]

Mark Geneformer tests to disable them by default, and avoid installing Geneformer in CI due to dependency issues (#1378).

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.89%. Comparing base (5817276) to head (69b8e23).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...sts/experimental/ml/huggingface/test_geneformer.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1381      +/-   ##
==========================================
- Coverage   91.94%   91.89%   -0.06%     
==========================================
  Files          83       80       -3     
  Lines        6742     6601     -141     
==========================================
- Hits         6199     6066     -133     
+ Misses        543      535       -8     

Flag	Coverage Δ
unittests	91.89% <80.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mlin]

@ivirshup updated as we discussed, with deprecation notices and still-fewer dependencies. I will make a separate PR next week to add the code & tests to tools/models/geneformer.