[Bug] [Relay] Error when compiling ONNX with LpPool

[luyaor]

Description

When compiling following model with TVM, it will error.

The model(with ONNX as frontend) with error is as follows, check bug.onnx in bug4.zip.

I think it is because ONNX allows attribute p in LpPool with a default value 2. Please check LpPool in ONNX: https://github.com/onnx/onnx/blob/master/docs/Operators.md#lppool

Error Log

Traceback (most recent call last):
  File "check.py", line 11, in <module>
    mod, params = relay.frontend.from_onnx(onnx_model, {})
  File "/Users/luyaor/Documents/tvm/python/tvm/relay/frontend/onnx.py", line 2806, in from_onnx
    mod, params = g.from_onnx(graph, opset, freeze_params)
  File "/Users/luyaor/Documents/tvm/python/tvm/relay/frontend/onnx.py", line 2613, in from_onnx
    op = self._convert_operator(op_name, inputs, attr, opset)
  File "/Users/luyaor/Documents/tvm/python/tvm/relay/frontend/onnx.py", line 2721, in _convert_operator
    sym = convert_map[op_name](inputs, attrs, self._params)
  File "/Users/luyaor/Documents/tvm/python/tvm/relay/frontend/onnx.py", line 712, in _impl_v1
    p = _expr.const(attr["p"], dtype)
KeyError: 'p'

How to reproduce

Environment

Python3, with tvm, onnx

tvm version: c31e338 Wed Dec 9 14:52:58 2020 +0900

1. Download
   bug4.zip

2. Run python check.py.

[luyaor]

I also created a corresponding PR for this issue: #7242

[luyaor]

Closed with #8866