Update opgen for it to be runnable (#1922)

Previously the opgen module assumes that it is installed as a package
and can be found at script import. Since we are now just running it as a
standalone script, I updated to imports so that it can be run directly.
Added a README.

opgen/README.md

@@ -0,0 +1,17 @@
+# Generator for onnx_opset
+
+Use this module the generate onnx_opset implementations when new opsets are introduced with new ONNX versions.
+
+## Generate
+
+```sh
+python opgen
+```
+
+Run
+
+```sh
+python opgen -h
+```
+
+for more information.

opgen/__main__.py

@@ -9,7 +9,7 @@
 import textwrap
 from pathlib import Path
 
-from opgen.onnx_opset_builder import (
+from onnx_opset_builder import (
     OpsetId,
     OpsetsBuilder,
     format_opsetid,

opgen/onnx_opset_builder.py

@@ -9,6 +9,7 @@
 from textwrap import dedent
 from typing import Annotated, Any, Iterable, Optional, Set, TextIO
 
+import pygen as cg
 from onnx.defs import (
     AttributeProto,
     OpSchema,
@@ -17,8 +18,6 @@
 )
 from onnx.helper import get_attribute_value
 
-import opgen.pygen as cg
-
 __all__ = [
     "OpsetId",
     "parse_opsetid",