[MRG] Fix UnbalancedSinkhornTransport transform failing on nx.array_equal (closes #650)

[JaySingh79]

fit() now initializes the backend attribute nx before the check_params
gate, matching the behavior of BaseTransport.fit. Previously, a fit call
with missing parameters left self.nx as None, causing any subsequent
transform() call to crash with:
AttributeError: 'NoneType' object has no attribute 'array_equal'

Adds a non-regression test covering both incomplete fit and a separate
transform call.

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.86%. Comparing base (2dad755) to head (b5e03cc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #837   +/-   ##
=======================================
  Coverage   96.85%   96.86%           
=======================================
  Files         128      128           
  Lines       25923    25938   +15     
=======================================
+ Hits        25109    25126   +17     
+ Misses        814      812    -2     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[rflamary]

Hello @JaySingh79 thanks for the PR, this is a nice fix, could you please add the information in the RELEASE.md file in the Bug fix section (with referencexs to the Issue and PR)

[JaySingh79]

Sure *Jay Singh* *Final Year Undergraduate | * *IIT Kharagpur* Connect: LinkedIn <https://www.linkedin.com/in/jay-singh-736825227/> | +91-8651274328 <https://wa.me/918651274328> Work: Portfolio <https://jaysingh.vercel.app> | Github <https://github.com/JaySingh79>

…

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[JaySingh79]

Hello @JaySingh79 thanks for the PR, this is a nice fix, could you please add the information in the RELEASE.md file in the Bug fix section (with referencexs to the Issue and PR)

Hey,
Added the entry to RELEASES.md under 0.9.8 - also closed the issues for UnbalancedSinkhornTransport transform failing on nx.array_equal (Issue #650) as requested. Let me know, if anything else is required.