[onnx] Show warning when unused inputs are removed

[xuzijian629]

With current behavior, the saved input_0.pb, ..., input_{n-1}.pb input tensors can have different number than the number of model inputs. This may confuse users.

I think removing unnecessary inputs is fine.

https://github.com/pfnet/pytorch-pfn-extras/blob/157b16004bd5654350fa34d43472d313b4a423a7/pytorch_pfn_extras/onnx/export_testcase.py#L336-L341