fix(jittor): use native checkpoints in NAS example

[moriyaya]

Summary

• save NAS checkpoints in Jittor-native .pkl format;
• remove the duplicate per-epoch save;
• keep checkpoint loading compatible with jt.load.

Rationale

Using a .pt filename selects Jittor's PyTorch-format serializer, which imports PyTorch. A clean BOAT-jit environment therefore completes NAS training but fails while saving the checkpoint. Native .pkl serialization removes this unintended dependency.

Validation

• syntax-checked the modified NAS Python files;
• confirmed that the epoch loop has one weights.pkl save and that checkpoint helper filenames use .pkl;
• Jittor was unavailable in the local environment, so the native save/load round-trip and CUDA NAS run were not executed;
• confirmed that the PR contains no unrelated changes.