Expose OpBuilder as onnxscript.OpBuilder (#2840)

Add OpBuilder to __all__ and import it from _internal.builder in the
top-level onnxscript package.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gramalingam

onnxscript/__init__.py

@@ -13,6 +13,7 @@
     "OnnxFunction",
     "TracedOnnxFunction",
     "GraphBuilder",
+    "OpBuilder",
     "proto2python",
     "external_tensor",
     "BFLOAT16",
@@ -129,7 +130,7 @@
 # isort: on
 
 from . import ir, nn, optimizer, rewriter, version_converter
-from ._internal.builder import GraphBuilder
+from ._internal.builder import GraphBuilder, OpBuilder
 from ._internal.utils import external_tensor
 from ._internal.values import OnnxFunction, TracedOnnxFunction