Remove dlpark dependency and implement DLPack directly

[isPANN]

Background

The dlpark crate (used for DLPack tensor interop) uses PyO3's deprecated IntoPy trait, causing deprecation warnings:

warning: use of deprecated method `pyo3::IntoPy::into_py`: `IntoPy` is going to be replaced by `IntoPyObject`.

Rationale for Removal

• dlpark is maintained by an individual developer
• No guarantee of timely updates to PyO3 0.23+ API
• DLPack protocol is straightforward enough to implement directly

Plan

1. Implement DLPack capsule creation using PyO3's PyCapsule API directly
2. Remove dlpark from dependencies
3. Maintain zero-copy GPU tensor interop with PyTorch

References

• DLPack spec: https://github.com/dmlc/dlpack
• PyO3 PyCapsule: https://docs.rs/pyo3/latest/pyo3/types/struct.PyCapsule.html