Mistake in `output_dir` and `overwrite` parameters validation

[Aivean]

This assert only lets in the following combination of parameter values:

• output_dir must exist and overwrite=False

https://github.com/fumihwh/onnx-pytorch/blob/2902b4d7cfb0470e2a91e29595ba2dfb8dcee50f/onnx_pytorch/code_gen.py#L317-L319

From the context is seems that other combinations of parameter values should also be valid, such as:

• output_dir doesn't exist an overwrite is whatever
• output_dir exist and overwrite=True

I think the fix is to change the clause to:

assert not os.path.exists(
     output_dir 
 ) or overwrite, f"{output_dir} is not empty and overwrite is not True." 

[fumihwh]

@Aivean Sorry for late reply. Some PR will resolve this issue.