[Code scan] Initialize EigLoss windowed loss when no bands fall inside the energy window

[njzjz]

This issue is part of a Codex global repository scan.

Problem:
In EigLoss.forward, loss is assigned only if mask_in contains at least one selected value. If the configured energy window excludes all selected bands but mask_out contains entries, the code evaluates loss + ... before loss exists.

Code reference:
https://github.com/deepmodeling/deeptb/blob/86c60c73996f0dd961c3138f2e88424382cb734e/dptb/nnops/loss.py#L215-L219

Impact:
Valid narrow-window configurations can fail with UnboundLocalError during training or evaluation.

Suggested fix:
Initialize loss before the mask branches, or compute in-window and out-of-window terms independently before summing the terms that are present.