toStableHlo

Export a ComputeGraph into a minimal StableHLO MLIR module text.

Notes:

• This function now uses the new modular converter architecture for better extensibility.

• It maintains backward compatibility with the original implementation.

• Currently supports: input, add, matmul, relu. Unsupported ops are emitted as comments.

• DType mapping expects TensorSpec.dtype to be strings like "FP32", "F32", "F64", "I32".