fix(metrics): correct compute() return values to prevent ValueError crash

[Aamod-Dev]

What type of PR is this?
/kind bug

What this PR does / why we need it:
This PR fixes a bug in core/testcasecontroller/metrics/metrics.py that causes a ValueError: not enough values to unpack (expected 3, got 2) when running lifelong learning or incremental learning benchmarks with a malformed (non-square) task matrix.

Previously, when the compute() function detected a malformed matrix (flag=False), it returned a 2-tuple: (bwt_score, fwt_score). However, all callers of this function (e.g. bwt_func, fwt_func, matrix_func) unconditionally expect a 3-tuple (_, BWT_score, _ = compute(...)), resulting in an immediate crash.

This commit updates the error path of compute() to consistently return a 3-tuple (None, np.nan, np.nan) when flag=False, satisfying caller expectations.

Which issue(s) this PR fixes:
Fixes #531

Special notes for your reviewer:
N/A

[kubeedge-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Aamod007
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[kubeedge-bot]

Welcome @Aamod007! It looks like this is your first PR to kubeedge/ianvs 🎉

[gemini-code-assist]

Code Review

This pull request updates the compute function in core/testcasecontroller/metrics/metrics.py to return a 3-tuple (None, np.nan, np.nan) instead of a 2-tuple of np.nan values when the flag condition is not met. There are no review comments, so I have no feedback to provide.

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[Aamod-Dev]

/assign @MooreZheng

[Aamod-Dev]

/assign @jaypume