[Model] Enable LoRA support for tower and connector in Llama Nemotron VL

[aktech]

Purpose

Part of #31479: enable LoRA on the Llama Nemotron VL vision tower + connector.

• Implements get_num_mm_encoder_tokens() / get_num_mm_connector_tokens() for LlamaNemotronVLChatModel
• Fixes startup crash: with enable_tower_connector_lora=True the tower token budget resolved to None, failing punica wrapper allocation with TypeError: empty(): argument 'size' must be tuple of ints, not NoneType
• Token math differs from InternVL: vision features have no CLS token (extract_feature reshapes the full sequence to an h x w grid), so the encoder sees exactly (image_size // patch_size)^2 tokens per tile and the connector sees num_image_token tokens per tile after pixel shuffle

Test Plan

pytest tests/models/multimodal/processing/test_nemotron_vl.py -k mm_lora
pytest tests/lora/test_nemotron_vl.py   # serves the real 8B model, needs 1 GPU

Test Result

Unit tests: 5 passed.
E2E: 1 passed on RTX 8000 serving nvidia/Llama-3.1-Nemotron-Nano-VL-8B-V1 with enable_tower_connector_lora=True; image descriptions correct. The same test on main crashes at engine init with the TypeError above.

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AI assistance: this PR was developed with assistance from Claude Code (Opus 5). All changes were reviewed line-by-line by the author, and all tests were run and verified by the author.

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