Fix WebGPU reduction kernels for empty tensor inputs
Add proper empty-set identity values to WebGPU reduction shader
generation. Previously, the empty input path used loop_header_/
loop_footer_ which referenced undefined variables (first_element
for Max/Min) or divided by zero (ReduceMean).
Changes:
- Add reduce_op_empty_identity_map with WGSL expressions for each
reduction op's ONNX identity value (Max→-inf, Min→+inf, Sum→0,
Prod→1, Mean→0, etc.)
- Modify ReduceNaiveProgram::GenerateShaderCode to directly output
the identity value for empty inputs instead of running broken
loop header/footer code
- Enable allow_empty_input for ReduceMax and ReduceMin WebGPU
kernels (was false, causing CheckInput to reject empty tensors)
- Remove WebGPU from empty tensor test exclusion lists
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Justin Chu <justinchu@microsoft.com>
