Allow opset_version to be set explicitly when exporting (#2615)

I think it would be nice to explicitly set opset_version when exporting,
particularly when a custom/particular Opset is being used and the
default opset can't be inferred.

Example:

```py
from onnxscript import script
from onnxscript import opset15 as op
from onnxscript.values import Opset
import numpy as np
from onnxscript import STRING
from onnxruntime import InferenceSession


ai_onnx = Opset("ai.onnx.ml", version=2)

@script(ai_onnx, default_opset = op)
def label_encoder(X: STRING["D"]):
    Y = ai_onnx.LabelEncoder(X, keys_strings=["a", "b", "c"], values_int64s=[0, 1, 2], default_int64=42)
    # Y = Y + 0.0 # to force opset version downgrade
    return Y


print(label_encoder(np.array(["a", "b", "c"])))
session = InferenceSession(label_encoder.to_model_proto(ir_version=10).SerializeToString())

for key, value in {"a": 0, "b": 1, "c": 2}.items():
    assert label_encoder(np.array([key]))[0] == value
    assert session.run(None, {"X": np.array([key])})[0] == value

```

This currently errors with 

```sh
Traceback (most recent call last):
  File "/Users/XXX/Development/projects/jet/test_onnxscript_label.py", line 25, in <module>
    session = InferenceSession(label_encoder.to_model_proto(ir_version=10).SerializeToString())
  File "/Users/XXX/Development/projects/jet/.venv/lib/python3.10/site-packages/onnxruntime/capi/onnxruntime_inference_collection.py", line 472, in __init__
    self._create_inference_session(providers, provider_options, disabled_optimizers)
  File "/Users/XXX/Development/projects/jet/.venv/lib/python3.10/site-packages/onnxruntime/capi/onnxruntime_inference_collection.py", line 552, in _create_inference_session
    sess = C.InferenceSession(session_options, self._model_bytes, False, self._read_config_from_model)
onnxruntime.capi.onnxruntime_pybind11_state.Fail: [ONNXRuntimeError] : 1 : FAIL : /Users/runner/work/1/s/onnxruntime/core/graph/model_load_utils.h:56 void onnxruntime::model_load_utils::ValidateOpsetForDomain(const std::unordered_map<std::string, int> &, const logging::Logger &, bool, const std::string &, int) ONNX Runtime only *guarantees* support for models stamped with official released onnx opset versions. Opset 23 is under development and support for this is limited. The operator schemas and or other functionality may change before next ONNX release and in this case ONNX Runtime will not guarantee backward compatibility. Current official support for domain ai.onnx is till opset 22.
```

To force it to work in the current state, one would have to do:

```py
@script(ai_onnx, default_opset = op)
def label_encoder(X: STRING["D"]):
    Y = ai_onnx.LabelEncoder(X, keys_strings=["a", "b", "c"], values_int64s=[0, 1, 2], default_int64=42)
    Y = Y + 0.0 # to force opset version downgrade/inserted from the to_model_proto call
    return Y
```

To force the opset to be downgraded, since the `default_opset` is never
called.

Happy to be challenged if there is a better way. I can imagine something
weird/unintended might occur if the user sets `default_opset` to
something other than what is defined in `@script(...,
default_opset=<op>)` but that generally shouldn't be a problem?

Author: NoRaincheck

onnxscript/irbuilder.py

@@ -321,6 +321,7 @@ def to_model_proto(
         input_types: Optional[Sequence[ONNXType]] = None,
         output_types: Optional[Sequence[ONNXType]] = None,
         value_infos: dict[str, ONNXType] | None = None,
+        opset_version: int | None = None,
         **kwargs,
     ) -> onnx.ModelProto:
         """Converts this instance into a `onnx.ModelProto`.
@@ -336,6 +337,8 @@ def to_model_proto(
                 are set to be of the corresponding type in this list.
             value_infos: A dictionary mapping intermediate variable names to ONNX types.
                 Used to set value_info for intermediate variables.
+            opset_version: The standard opset version to use for the model if it
+                cannot be inferred. Otherwise defaults to the current opset version.
             kwargs: Additional parameters given to function :func:`onnx.helper.make_model`.
 
         Returns:
@@ -393,8 +396,8 @@ def to_proto(f):
 
         if "" not in opsets:
             # No operator is using the standard opset.
-            # A default value is given.
-            opsets[""] = onnx_opset_version()
+            # Use the specified version if provided or the default value.
+            opsets[""] = opset_version if opset_version is not None else onnx_opset_version()
 
         if "ir_version" not in kwargs:
             kwargs["ir_version"] = select_ir_version(opsets[""])