onnxruntime

DML EP Register Split18 (#15931)

Register Split18 for DirectML

Split13 was previously implemented. Split18 adds a new attribute called
"num_outputs" that must be used mutually exclusively with the "split"
input.

The "num_outputs" attribute wil split the tensor evenly (and handles odd
uneven splits). To implement, the DML split tensor just needs to be
overridden in the presence of the num_output attribute.

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Co-authored-by: Dwayne Robinson <dwayner@microsoft.com>