Support current gradient checkpointing APIs

[zjn20030811]

Summary

• Support both per-module and model-level gradient-checkpointing hook signatures used by diffusers releases.
• Propagate the checkpoint function supplied by newer releases to checkpointable submodules.
• Add a small CPU regression test that enables and disables checkpointing through NextDiTCrossAttn.

Problem

LuminaNextDiT2DModel implemented the older
_set_gradient_checkpointing(module, value=False) hook. Newer diffusers
calls the hook with enable= and gradient_checkpointing_func=, so the
default InternVLA-N1 configuration failed during model construction whenever
gradient checkpointing was enabled.

The adapter accepts both conventions while preserving the existing behavior
on older releases.

Validation

• PYTHONPATH=F:\\contributor\\.deps-wan-audit;F:\\contributor\\audit-internnav-next2 python -m pytest tests/unit_test -q (5 passed, 1 skipped)
• The focused regression passes with diffusers 0.35.2 and 0.32.1.
• python -m py_compile internnav/model/basemodel/internvla_n1/nextdit_traj.py tests/unit_test/test_gradient_checkpointing_compat.py
• git diff --check