More robust ONNX load

• Detect unknown attributes in AveragePool, Conv and MaxPool.
• Bug correction in handling of auto_pad attribute.
• Added has_native_coverage() and native_coverage_report() functions.

The native_coverage_report() outputs a report like this:

Native operators: 588 (6 types)
- Add: 30
- BatchNorm: 72
- Conv: 37
- LeakyReLU: 72
- PaddedConv: 38
- Producer: 339
Generic operators: 220 (20 types)
- Cast: 33
- Ceil: 7
- Concat: 20
- Div: 19
- Exp: 3
- Gather: 3
- Loop: 6
- Mul: 17
- NonMaxSuppression: 1
- ReduceMin: 1
- Reshape: 15
- Resize: 2
- Shape: 3
- Sigmoid: 9
- Slice: 43
- Squeeze: 7
- Sub: 13
- Tile: 6
- Transpose: 4
- Unsqueeze: 8
Native types coverage: 23.1% (6/26)
Native operators coverage: 72.8% (588/808)