Generate name for each member of list arg

[xuzijian629]

Problem

ppe.onnx.export_testcase fails with list input. This is because the number of input_names is incorrect. The number of input tensors in the exported onnx is total tensor numbers, not len(args).

Reproduction

import torch
import pytorch_pfn_extras.onnx as tou
import torch.onnx.symbolic_helper
import torch.onnx.symbolic_registry

@torch.onnx.symbolic_helper.parse_args("v", "b", "f")
def pad_sequence(g, input, batch_first, padding_value):
    assert batch_first, "batch_first=False is not supported"
    ret = g.op("org.chainer::ChainerSequencePad", input, value_f=padding_value)
    return ret

for opset in range(9, 16):
    torch.onnx.symbolic_registry.register_op("pad_sequence", pad_sequence, "", opset)

class Model(torch.nn.Module):
    def forward(self, xs):
        return torch._C._nn.pad_sequence(xs, True, 0)


model = Model()
args = ([torch.rand(2, 5), torch.rand(3, 5)],)
torch.onnx.export(model, args, "test.onnx")  # Success!!
tou.export_testcase(model, args, "test")  # Fails!!

Error:

Traceback (most recent call last):
  File "hoge.py", line 36, in <module>
    tou.export_testcase(model, args, "test", use_pfto=False)
  File "/mnt/vol21/joe/pfvm/third_party/pytorch-pfn-extras/pytorch_pfn_extras/onnx/export_testcase.py", line 303, in export_testcase
    used_input_index_list.append(input_names.index(used_input.name))
ValueError: 'onnx::SequenceConstruct_1' is not in list

torch version: 1.12.0

This PR

Unrolls the input list args and generate names for all tensors. For example, if the input args is args=([[a,b],c], d), the generated input names are

[input_0_0_0, input_0_0_1, input_0_1, input_1]

whereas current master generates names as

[input_0, input_1]

[xuzijian629]

/test

[xuzijian629]

todo: add test

[xuzijian629]

Update: I summarized the problem of extending current ppe.onnx.export for models with list inputs at https://github.com/pfnet/pytorch-pfn-extras/issues/572.

In short, torch.onnx.export automatically unrolls list inputs in their internal trace API (torch.jit._get_trace_graph). However, torch's public API for trace (torch.jit.trace) does not unroll list inputs. So maybe we should create a wrapper class for export that unroll list inputs. How do you think about this?

[xuzijian629]

With 6c732649a7ffda442d9c3beaf12427f07e2f0375, onnx export with use_pfto=False works well (because then pee.onnx.export will delegate to torch.onnx.export and list inputs are automatically unrolled, so it makes sense to unroll inputs to generate names).

Note that with use_pfto=True, currently, exported onnx has single list input (that is sequence typed in ONNX) and we MUST NOT unroll input args.

[xuzijian629]

My idea: always unroll args to follow torch.onnx.export style. For PFTO, we wrap the module for list inputs (future work).

[xuzijian629]

memo: Also support list, tuple outputs