Implement get_c_interface method for Kokkos plugin (#837)

**Context:** Catalyst is removing the Kokkos plugin from its code base
(PennyLaneAI/catalyst#974) and is attempting to
use it directly from Lightning. However, the C interface was not
provided via Python.

**Description of the Change:** Implement the C interface provider method
in Python.

**Benefits:** Catalyst can use the Kokkos plugin.

---------

Co-authored-by: ringo-but-quantum <[email protected]>

.github/CHANGELOG.md

@@ -62,6 +62,7 @@
 
 * Add a Catalyst-specific wrapping class for Lightning Kokkos.
   [(#770)](https://github.com/PennyLaneAI/pennylane-lightning/pull/770)
+  [(#837)](https://github.com/PennyLaneAI/pennylane-lightning/pull/837)
 
 * Add `initial_state_prep` option to Catalyst TOML file.
   [(#826)](https://github.com/PennyLaneAI/pennylane-lightning/pull/826)

.github/workflows/tests_lkcpu_python.yml

@@ -246,7 +246,8 @@ jobs:
         run: |
           cd main/
           DEVICENAME=`echo ${{ matrix.pl_backend }} | sed "s/_/./g"`
-            PL_DEVICE=${DEVICENAME} python -m pytest tests/ $COVERAGE_FLAGS --splits 7 --group ${{ matrix.group }} \
+            # Remove `python -m` to avoid running tests with relative modules
+            PL_DEVICE=${DEVICENAME} pytest tests/ $COVERAGE_FLAGS --splits 7 --group ${{ matrix.group }} \
               --store-durations --durations-path='.github/workflows/python_lightning_kokkos_test_durations.json' --splitting-algorithm=least_duration
             mv .github/workflows/python_lightning_kokkos_test_durations.json ${{ github.workspace }}/.test_durations-${{ matrix.exec_model }}-${{ matrix.group }}
           pl-device-test --device ${DEVICENAME} --skip-ops --shots=20000 $COVERAGE_FLAGS --cov-append
@@ -297,4 +298,4 @@ jobs:
         with:
           fail_ci_if_error: true
           verbose: true
-          token: ${{ secrets.CODECOV_TOKEN }}
+          token: ${{ secrets.CODECOV_TOKEN }}

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.38.0-dev25"
+__version__ = "0.38.0-dev26"

pennylane_lightning/lightning_kokkos/lightning_kokkos.py

@@ -17,6 +17,8 @@
 interfaces with C++ for fast linear algebra calculations.
 """
 
+import os
+import sys
 from os import getenv
 from pathlib import Path
 from typing import List
@@ -834,3 +836,45 @@ def processing_fn(tape):
                 return self.adjoint_jacobian(new_tape, starting_state, use_device_state)
 
             return processing_fn
+
+    @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.
+        """
+
+        # The shared object file extension varies depending on the underlying operating system
+        file_extension = ""
+        OS = sys.platform
+        if OS == "linux":
+            file_extension = ".so"
+        elif OS == "darwin":
+            file_extension = ".dylib"
+        else:
+            raise RuntimeError(
+                f"'LightningKokkosSimulator' shared library not available for '{OS}' platform"
+            )
+
+        lib_name = "liblightning_kokkos_catalyst" + file_extension
+        package_root = Path(__file__).parent
+
+        # The absolute path of the plugin shared object varies according to the installation mode.
+
+        # Wheel mode:
+        # Fixed location at the root of the project
+        wheel_mode_location = package_root.parent / lib_name
+        if wheel_mode_location.is_file():
+            return "LightningKokkosSimulator", wheel_mode_location.as_posix()
+
+        # Editable mode:
+        # The build directory contains a folder which varies according to the platform:
+        #   lib.<system>-<architecture>-<python-id>"
+        # To avoid mismatching the folder name, we search for the shared object instead.
+        # TODO: locate where the naming convention of the folder is decided and replicate it here.
+        editable_mode_path = package_root.parent.parent / "build"
+        for path, _, files in os.walk(editable_mode_path):
+            if lib_name in files:
+                lib_location = (Path(path) / lib_name).as_posix()
+                return "LightningKokkosSimulator", lib_location
+
+        raise RuntimeError("'LightningKokkosSimulator' shared library not found")

tests/test_device.py

@@ -107,3 +107,49 @@ def circuit():
         return qml.state()
 
     assert np.allclose(circuit(), np.array([1.0]))
+
+
+@pytest.mark.skipif(
+    (device_name != "lightning.kokkos" or sys.platform != "win32"),
+    reason="This test is for Kokkos under Windows only.",
+)
+def test_unsupported_windows_platform_kokkos():
+    """Test unsupported Windows platform for Kokkos."""
+
+    dev = qml.device(device_name, wires=0)
+
+    with pytest.raises(
+        RuntimeError,
+        match="'LightningKokkosSimulator' shared library not available for 'win32' platform",
+    ):
+        dev.get_c_interface()
+
+
+@pytest.mark.skipif(
+    (device_name != "lightning.kokkos" or sys.platform != "linux"),
+    reason="This test is for Kokkos under Linux only.",
+)
+def test_supported_linux_platform_kokkos():
+    """Test supported Linux platform for Kokkos."""
+
+    dev = qml.device(device_name, wires=0)
+
+    dev_name, shared_lib_name = dev.get_c_interface()
+
+    assert dev_name == "LightningKokkosSimulator"
+    assert "liblightning_kokkos_catalyst.so" in shared_lib_name
+
+
+@pytest.mark.skipif(
+    (device_name != "lightning.kokkos" or sys.platform != "darwin"),
+    reason="This test is for Kokkos under MacOS only.",
+)
+def test_supported_macos_platform_kokkos():
+    """Test supported MacOS platform for Kokkos."""
+
+    dev = qml.device(device_name, wires=0)
+
+    dev_name, shared_lib_name = dev.get_c_interface()
+
+    assert dev_name == "LightningKokkosSimulator"
+    assert "liblightning_kokkos_catalyst.dylib" in shared_lib_name