[onert] Incorrect use of Pad data in cpu backend

[ragmani]

What

As we discussed offline, In circle schema, pad data of Pad operation is a tensor, not PadOption. So pad data must be used as a tensor instead of constant data in cpu backend.

Originally posted by @ragmani in #12535 (comment)

/cc @YongseopKim

[YongseopKim]

While #12413, I'll fix it.

[YongseopKim]

I've instigated so far, I figure out that pad op's pad data is used as const in many tests.

An example.

// Pad.test.cc
TEST_P(PadVariation, Test)
{
  auto &param = GetParam();

  CircleGen cgen;
  int in = cgen.addTensor({{1, 2, 2, 1}, param.data_type}, param.scale, param.zero_point);
  std::vector<int32_t> padding_data{0, 0, 1, 1, 1, 1, 0, 0};
  uint32_t padding_buf = cgen.addBuffer(padding_data);
  int padding = cgen.addTensor({{4, 2}, circle::TensorType::TensorType_INT32, padding_buf}); // **HERE**
  int out = cgen.addTensor({{1, 4, 4, 1}, param.data_type}, param.scale, param.zero_point);

  cgen.addOperatorPad({{in, padding}, {out}});
  cgen.setInputsAndOutputs({in}, {out});
  _context = std::make_unique<GenModelTestContext>(cgen.finish());
  _context->addTestCase(param.tcd);
  _context->setBackends({"acl_cl", "acl_neon", "cpu"});

  SUCCEED();
}
// Other tests also handle pad data as const data

The best way I think now is to support both tensor and const.

Feel free to write comments if I misunderstand.

[YongseopKim]

Done