onnx-mlir/doc/ImportONNXDefs.md

# Import ONNX specifications into ONNX MLIR
The specifications of ONNX are defined under onnx/defs directory in ONNX projects. 
There is a python script onnx/defs/gen_doc.py that automatically generate documents about operations in ONNX (docs/Operations.md). 
ONNX MLIR modified this script to import ONNX specifications into ONNX MLIR. There are two files generated for ONNX MLIR with the modified gen_doc.py:
1. src/Dialect/ONNX/ONNXOps.td.inc: Operation defintion for MLIR tablegen. Will be included in src/Dialect/ONNX/ONNXOps.td
2. src/Builder/OpBuildTable.inc: c code for ONNX MLIR frontend to import operation nodes from ONNX model. Will be included in src/Builder/FrontendDialectTransformer.cpp

## How to use the script
1. Get ONNX. You can use onnx-mlir/third_party/onnx
2. Perform the following steps (assume that we use onnx-mlir/third_party/onnx):
```bash
$ cd onnx-mlir
$ cp doc/gen_doc.py third_party/onnx/onnx/defs/
$ cd third_party/onnx
$ python onnx/defs/gen_doc.py
$ cd ../..
$ cp third_party/onnx/onnx/defs/ONNXOps.td.inc src/Dialect/ONNX/
$ cp third_party/onnx/onnx/defs/OpBuildTable.inc src/Builder/
```

## Consistency
The Operators.md generated by gen_doc.py is copied into doc. Please refer to this specification, not the one in onnx github, to make sure operators are consistent in version with ONNXOps.td.inc.

## Customization
In addition to following the ONNX specification, the modified gen_doc.py provides some mechanism for you to customize the output. 
Several tables are defined at the beginning of the script:
1. special_attr_defaults: gives attribute special default value.
2. special_op_handler: creates special import function in frontend_dialect_transformer.cpp. Currently special handler is used for operations with oprational arguments
3. OpsWithShapeInference: list of operations which have shape inference defined
4. OpsWithCanonicalizer: list of operations which have canonical form
5. OpsWithPromotableConstOperands: list of operations which have operands that, if produced by constant operations, should be promoted to become an attribute (via attribute promotion)
6. custom_builder_ops_list: list of operations which need custom build methods to deduce result types