Add type inference for BroadcastGradientArgs (#5501)
* Add type inference for BroadcastGradientArgs
This change enables the ONNX shape and type inference to work on a function body containing a BroadcastGradientArgs op. Without this change, the dummy inference function is used, and no types are inferred for the output here:
https://github.com/onnx/onnx/blob/531e6dd459003fc8d13b8abb66b29a72a571c865/onnx/shape_inference/implementation.cc#L467-L469
* Handle optional outputs.
