Checkpoint interaction neighbor normalization

[darthjaja6]

Fixes #1415.

Summary

• register each interaction block avg_num_neighbors as a persistent buffer, so it follows checkpoints and dtype/device conversions
• keep strict loading compatible with legacy checkpoints that do not contain the new buffer
• update model conversion and fine-tuning transfers to copy legacy float values safely

Validation

• pytest tests/unit/test_finetuning_utils.py tests/unit/test_models.py -q (15 passed)
• pytest tests/unit -q -k "not benchmark" (247 passed, 16 deselected)
• MACE_REQUIRE_CAPS=gpu,cueq,oeq pytest tests/backends -q (50 passed)
• converted the released legacy MH1 model and verified the float normalization becomes a checkpointed tensor
• black, isort, pylint, and git diff --check