feat: add Einsum op, WithConstantVariables, and bug fixes

[ajroetker]

Summary

New Features

• Einsum op: Add support for ONNX Einsum with 2 operands
• WithConstantVariables: New option to embed all model variables as graph constants, required for models that use variables in ops needing compile-time values (e.g., Range for rotary embeddings)
• DynamicDim support: Propagate DynamicDim through broadcast and slice ops — detect DynamicDim axes, preserve axis names for the specialization system, and fall through to existing concrete-dimension logic when all dims are static

Bug Fixes

• CumSum: Use adjustedAxis consistently (CumSum supports negative axes but other calls in the same function were using the raw axis)
• RotaryEmbedding: Add bounds check requiring at least 4 inputs before accessing inputs[2] and inputs[3]
• GroupQueryAttention: Add rank check before accessing Dimensions[2] on past KV inputs to prevent index-out-of-range on malformed tensors
• LayerNormalization: Only allocate biasShape when bias is non-nil, and use bias's own dimensions instead of scale dimensions
• ScatterND: Fix error message printing data rank instead of indices rank
• ConstantOfShape: Make value attribute optional per ONNX spec, defaulting to scalar float32 zero when absent
• LSTM: Fix copy-paste bug reading activation_alpha for activationBeta instead of activation_beta; fix activaitons typo
• MultiHeadAttention: Fix 2D mask reshape producing (1,batch,kv_seq,1) instead of (batch,1,1,kv_seq) by using Reshape directly
• convertIf: Use m.onnxWhere for dtype promotion between then/else branches instead of raw Where which panics on mismatched dtypes; panic with descriptive message when If node produces no outputs instead of returning nil
• QLinearMatMul: Fix nil pointer dereference when yZeroPoint is nil (.DType() was called before the nil check); collapse redundant if/else-if branches for zero-point subtraction
• convertSubGraph: Fix overwriting shared model maps (nodeOutputToNode, variableNameToValue) without saving/restoring originals — sub-graph names colliding with main graph names could corrupt state; thread *context.Context through so sub-graph nodes can resolve model variables correctly
• materialize.go: Fix varDesc = append(opsDesc, ...) copy-paste bug that left opsDesc always empty and overwrote varDesc in error messages
• prettyprint: Fix format verb for shapes.Shape (%d → %v)

Simplifications

• Remove redundant savedVariableExists/savedNodeExists maps — use Go's comma-ok idiom on the saved maps directly since nil is never a valid value

Test plan

• Existing tests pass
• Test Einsum with 2-operand models
• Test WithConstantVariables with rotary embedding models
• Test models that exercise convertIf/convertSubGraph paths