Trinity contrib - Fix notebook for upstream clone and add LNC documentation

[jimburtoft]

Fix for Trinity contrib demonstration notebook -- no model code changes.

• Fix nki-library install: use clone+copy workaround (pip install fails due to setuptools_scm issue)
• Fix HF_TOKEN: replace Python syntax in %%bash cell with proper bash
• Fix sys.path: point to /home/ubuntu/nxdi/ (upstream clone) not nxdi-fork
• Fix Nano TP: change from TP=2 to TP=1 (TP=2 invalid with LNC=2)
• Add kernel restart warnings between model sections (NeuronCore OOM)
• Add LNC configuration section to README with valid TP degrees
• Update validated results to match upstream testing (TP=1 Nano, +28% Mini)

Testing

I ran the notebook on a trn2.3xlarge with SDK 2.28

By submitting this PR, I confirm that:

• [x ] I have read and followed the contributing guidelines
• [ x] This is a community contribution and may have limited testing compared to officially-supported models
• [ x] The code follows best practices and is well-documented
• [ x] All required components listed above are included