Document ONNX operator contracts for TensorRT

[Copilot]

ONNX node semantics depend on the operator name, domain, imported opset, and schema. This adds Day 2 notes connecting those contracts to Mobius’s opset-24 requirements and TensorRT compatibility.

• Operator semantics

  • Explains domains, opset imports, schemas, and attributes.
  • Distinguishes ordered tensor inputs from node configuration.

• Mobius Attention contract

  • Identifies nonpad_kv_seqlen as tensor input index 6.
  • Identifies q_num_heads, kv_num_heads, scale, and is_causal as attributes.

• Opset lowering

  • Documents why TensorScatter and populated Attention input 6 prevent safe lowering to opset 23.

• TensorRT portability

  • Explains why checker-valid standard or vendor-domain nodes may remain unsupported by TensorRT.
  • Records the working-tree branch reference observed during inspection.

Attention(..., nonpad_kv_seqlen, q_num_heads=..., kv_num_heads=...)
               tensor input #6   attributes

• Fixes ONNX for TensorRT — Day 2: Inspect what TensorRT must parse #3