onnxruntime

Fix bug in handling of variadics in function schema creation (#15409)

### Description

The code handling variadic parameters when creating a schema for a
function has a minor bug.
The checking logic was nested inside a conditional, instead of being
outside.
Fix the logic, and add a test-case. This bugs manifests itself when the
first parameter in the
variadic list is not an input/output of the enclosing function.

### Motivation and Context

Fixes https://github.com/microsoft/onnxruntime/issues/15404

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Signed-off-by: Ganesan Ramalingam <grama@microsoft.com>