Expose build_function and build_graph in public API (#2913)

Add build_function and build_graph to onnxscript's top-level exports,
allowing downstream packages (e.g. mobius) to import them directly as
onnxscript.build_function / onnxscript.build_graph instead of reaching
into onnxscript._internal.builder.

Signed-off-by: G Ramalingam <grama@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gramalingam

onnxscript/__init__.py

@@ -16,6 +16,8 @@
     "OpBuilder",
     "OpBuilderBase",
     "TapeBuilder",
+    "build_function",
+    "build_graph",
     "proto2python",
     "external_tensor",
     "BFLOAT16",
@@ -132,7 +134,7 @@
 # isort: on
 
 from . import ir, nn, optimizer, rewriter, version_converter
-from ._internal.builder import GraphBuilder, OpBuilder
+from ._internal.builder import GraphBuilder, OpBuilder, build_function, build_graph
 from ._internal.tape_builder import OpBuilderBase, TapeBuilder
 from ._internal.utils import external_tensor
 from ._internal.values import OnnxFunction, TracedOnnxFunction