[NV] Add external distortion wrappers and tests

[frtrudel]

Summary

Add external distortion model wrappers and tests, enabling pluggable distortion models via a public ABC interface.

Changes

New modules

• gsplat/cuda/_torch_external_distortion.py — pure PyTorch reference implementation for external distortion
• gsplat/cuda/csrc/ExternalDistortionWrappers.cu — CUDA external distortion wrappers
• gsplat/cuda/include/ExternalDistortionWrappers.h — header for CUDA wrappers
• tests/test_external_distortion.py — tests for external distortion models

API

• Expose ExternalDistortionModelParameters as an ABC in gsplat/__init__.py for easier client-side integration
• Update rendering.py and _wrapper.py to support external distortion passthrough

Other

• Add copyright headers to new files (scoped to NV)

Test plan

• ./tests/run_tests.sh --3dgut tests/test_external_distortion.py — external distortion model tests