Fix unspecified mesh-axis integer validation

[huytransformer]

Description

Fixes fill_unspecified_mesh_axes by calling float.is_integer() when validating inferred mesh-axis parallelism.

Previously the assertion checked the bound method object instead of the boolean result, so non-integer inferred values could pass the intended validation.

Tests

• python3 -m py_compile src/maxtext/utils/max_utils.py tests/unit/max_utils_test.py
• git diff --check
• python3 -m pytest tests/unit/max_utils_test.py -k FillUnspecifiedMeshAxesTest

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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