Remove useless cast nodes in graph after converting to fp16 model

[xiaowuhu]

1. For ort 1.17, ORT will not remove cast(to16) automatically when meeting cast(to16)->cast(to32) pair. So we need remove this kind of pair in converter.
2. Due to security issue, onnx 1.15 and before cannot be used. So upgrade to 1.16 for all testing.