Check if onnx model already converted to fp16

[xiaowuhu]

Check if Onnx Model already be converted to fp16 before, by checking:

• if graph input, output has fp16 type
• if graph value_info has fp16 type
• if graph initializer has fp16 type
• if graph node exist cast to fp16 node.

If yes, raise exception. But use can use check_fp16_ready=False to skip this checking and force to convert the model to fp16 (maybe again).