Connect to OQD device runtime from Python layer (#1420)

**Context:**
A boilerplate Python OQD device was added in PR #1355. We want to verify
that the infrastructure introduced in that PR is sufficient to generate
OpenAPL in the runtime.

**Description of the Change:**
Added get_c_interface method for the OQD device to be able to point to
the c++ implementation of the device in python.
This makes qjit to accept an instance of the OQD device and be able to
connect to its runtime.

**Benefits:**
This allows for OQD runtime to be exposed to python, unlocking the next
steps which are implementing the llvm lowering and the runtime methods.

**Possible Drawbacks:**

**Related GitHub Issues:**
[sc-73709]

---------

Co-authored-by: Joey Carter <joseph.carter@xanadu.ai>

doc/releases/changelog-dev.md

@@ -12,6 +12,11 @@
 
 <h3>Internal changes ⚙️</h3>
 
+* The `get_c_interface` method has been added to the OQD device, which enables retrieval of the C++
+  implementation of the device from Python. This allows `qjit` to accept an instance of the device
+  and connect to its runtime.
+  [(#1420)](https://github.com/PennyLaneAI/catalyst/pull/1420)
+
 * `from_plxpr` now uses the `qml.capture.PlxprInterpreter` class for reduced code duplication.
   [(#1398)](https://github.com/PennyLaneAI/catalyst/pull/1398)
 
@@ -21,4 +26,6 @@
 
 This release contains contributions from (in alphabetical order):
 
-Christina Lee
+Christina Lee
+Mehrdad Malekmohammadi
+

frontend/catalyst/third_party/oqd/oqd_device.py

@@ -20,6 +20,7 @@
 trapped-ion quantum computer device.
 """
 from typing import Optional
+import platform
 
 from pennylane.devices import Device, ExecutionConfig
 from pennylane.transforms.core import TransformProgram
@@ -33,7 +34,18 @@ class OQDDevice(Device):
 
     config_filepath = get_lib_path("oqd_runtime", "OQD_LIB_DIR") + "/backend" + "/oqd.toml"
 
-    def __init__(self, wires, backend, shots, **kwargs):
+    @staticmethod
+    def get_c_interface():
+        """Returns a tuple consisting of the device name, and
+        the location to the shared object with the C/C++ device implementation.
+        """
+
+        system_extension = ".dylib" if platform.system() == "Darwin" else ".so"
+        lib_path = get_lib_path("oqd_runtime", "OQD_LIB_DIR") + "/librtd_oqd" + system_extension
+
+        return "oqd", lib_path
+
+    def __init__(self, wires, shots, backend="default", **kwargs):
         self._backend = backend
         _check_backend(backend=backend)
         super().__init__(wires=wires, shots=shots, **kwargs)

frontend/test/test_oqd/oqd/test_oqd_device.py

@@ -0,0 +1,69 @@
+# Copyright 2024 Xanadu Quantum Technologies Inc.
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for the OQD device.
+"""
+
+import pennylane as qml
+import pytest
+
+from catalyst.third_party.oqd import OQDDevice
+
+
+class TestOQDDevice:
+    """Test the OQD device python layer for Catalyst."""
+
+    def test_initialization(self):
+        """Test the initialization."""
+
+        device = OQDDevice(backend="default", shots=1000, wires=8)
+
+        assert device.backend == "default"
+        assert device.shots == qml.measurements.Shots(1000)
+        assert device.wires == qml.wires.Wires(range(0, 8))
+
+    def test_wrong_backend(self):
+        """Test the backend check."""
+        with pytest.raises(ValueError, match="The backend random_backend is not supported."):
+            OQDDevice(backend="random_backend", shots=1000, wires=8)
+
+    def test_execute_not_implemented(self):
+        """Test the python execute is not implemented."""
+        with pytest.raises(NotImplementedError, match="The OQD device only supports Catalyst."):
+            dev = OQDDevice(backend="default", shots=1000, wires=8)
+            dev.execute([], [])
+
+    def test_preprocess(self):
+        """Test the device preprocessing"""
+        dev = OQDDevice(backend="default", shots=1000, wires=8)
+        tranform_program, _ = dev.preprocess()
+        assert tranform_program == qml.transforms.core.TransformProgram()
+
+    def test_preprocess_with_config(self):
+        """Test the device preprocessing by explicitly passing an execution config"""
+        dev = OQDDevice(backend="default", shots=1000, wires=8)
+        execution_config = qml.devices.ExecutionConfig()
+        tranform_program, config = dev.preprocess(execution_config)
+        assert tranform_program == qml.transforms.core.TransformProgram()
+        assert config == execution_config
+
+    def test_get_c_interface(self):
+        """Test the device get_c_interface method."""
+        dev = OQDDevice(backend="default", shots=1000, wires=8)
+        name, _ = dev.get_c_interface()
+        assert name == "oqd"
+
+
+if __name__ == "__main__":
+    pytest.main(["-x", __file__])