Fix validation epoch metadata for visual backends

[dgexplores]

Summary

• include the validation epoch in the scalar payload when log_metric_by_epoch=True
• add regression coverage for the epoch-based validation logging path

This lets visualization backends such as WandbVisBackend define an epoch-based x-axis from the emitted scalar metadata. The scalar step remains unchanged.

Fixes #1692.

Validation

• pytest tests/test_hooks/test_logger_hook.py -q → 7 passed
• git diff --check → passed

The focused test run emitted existing PyTorch deprecation warnings only.

[CLAassistant]

All committers have signed the CLA.

[dgexplores]

Hi @nfcampos @hwchase17 @sydney-runkle,

This PR addresses mmengine issue for validation epoch metadata in visual backends (WandB).

Changes made:

• mmengine/hooks/logger_hook.py: Added tag['epoch'] = epoch line
• tests/test_hooks/test_logger_hook.py: Added regression test test_after_val_epoch

The fix ensures epoch data is properly captured during validation loops for visual backends.

Following OpenMMLab contributing guidelines, this PR is ready for review.

• Fix validates epoch metadata that was previously lost
• Test coverage added for sync validation loop
• Minimal changes - only 2 files modified (+7 total lines)

Thank you for reviewing!