[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.

pytorch-pfn-extras/pytorch_pfn_extras/onnx/export_testcase.py

Lines 336 to 341 in 157b160

	used_input_index_list = []
	for used_input in onnx_graph.graph.input:
	if used_input.name not in initializer_names:
	used_input_index_list.append(input_names.index(used_input.name))
	input_names = [input_names[i] for i in used_input_index_list]
	args = [args[i] for i in used_input_index_list]