## Problem
Models with fewer KV heads than nodes crash during tensor parallelism.
For example, Qwen3.5 MoE models have only 2 KV heads — trying to shard
across 4 nodes produces empty tensors and a reshape error at runtime.
The placement system already validates `hidden_size % num_nodes == 0`
but doesn't check KV heads, so it creates configurations that look valid
but blow up when the worker tries to split the attention heads.
Affected models include Qwen3.5-35B-A3B, Qwen3.5-122B-A10B,
Qwen3.5-397B-A17B, Qwen3-Next-80B-A3B, and Qwen3-Coder-Next (all have 2
KV heads).
## Changes
**Placement validation** (`src/exo/master/placement.py`):
- Combined KV heads divisibility check with the existing hidden_size
filter in a single pass
- Cycles where `num_key_value_heads % len(cycle) != 0` are now excluded
for tensor sharding
- Error message includes both constraints when no valid cycle is found
**Model card schema** (`src/exo/shared/models/model_cards.py`):
- Added optional `num_key_value_heads` field to `ModelCard` and
`ConfigData`
- Extracted from HuggingFace `config.json` (handles both top-level and
`text_config` nesting)
- Passed through in `fetch_from_hf()` for dynamically fetched cards
**All 68 inference model cards**
(`resources/inference_model_cards/*.toml`):
- Populated `num_key_value_heads` from each model's HuggingFace config
**Utility script** (`scripts/fetch_kv_heads.py`):
- Fetches `num_key_value_heads` from HuggingFace and updates TOML cards
- `--missing`: only fills in cards that don't have the field yet
- `--all`: re-fetches and overwrites everything
- Uses tomlkit for safe TOML editing and ThreadPoolExecutor for parallel
fetches
## Behavior
- Instance previews no longer show tensor options for models that can't
split their KV heads across the cluster size
- `place_instance()` rejects with a clear error instead of crash-looping
- Pipeline parallelism is unaffected
- 2-node tensor still works for 2-KV-head models (2 ÷ 2 = 1)
- Field is optional — existing custom cards without it continue to work
(validation is skipped when `None`)
