feat[cartesian]: gtc cuda backend deprecation (#1498)

## Description

GTC `cuda` backend was made available a few years ago for AI2 team
research. It has been kept updated but a recent poll shows that it is
not in use. Recent new features break the backend and we propose here to
hard deprecate it rather than keep spending time maintaining it.

`GT4PY_GTC_ENABLE_CUDA=1` can be used to force the use of the backend, but
will warn that any feature from February 2024 are not available/not
tested.

Additionally a mechanism to deprecate all GTC backends are now in use. Using
```python
@disabled(
    message="Disable message.",
    enabled_env_var="EnvVarToEnable",
)
```

## Requirements

- [x] All fixes and/or new features come with corresponding tests.

---------

Co-authored-by: Hannes Vogt <hannes@havogt.de>

src/gt4py/cartesian/backend/base.py

@@ -19,7 +19,21 @@
 import pathlib
 import time
 import warnings
-from typing import TYPE_CHECKING, Any, ClassVar, Dict, List, Optional, Protocol, Tuple, Type, Union
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Callable,
+    ClassVar,
+    Dict,
+    List,
+    Optional,
+    Protocol,
+    Tuple,
+    Type,
+    Union,
+)
+
+from typing_extensions import deprecated
 
 from gt4py import storage as gt_storage
 from gt4py.cartesian import definitions as gt_definitions, utils as gt_utils
@@ -39,7 +53,7 @@ def from_name(name: str) -> Optional[Type["Backend"]]:
     return REGISTRY.get(name, None)
 
 
-def register(backend_cls: Type["Backend"]) -> None:
+def register(backend_cls: Type["Backend"]) -> Type["Backend"]:
     assert issubclass(backend_cls, Backend) and backend_cls.name is not None
 
     if isinstance(backend_cls.name, str):
@@ -413,3 +427,28 @@ def build_extension_module(
         )
 
         return module_name, file_path
+
+
+def disabled(message: str, *, enabled_env_var: str) -> Callable[[Type[Backend]], Type[Backend]]:
+    # We push for hard deprecation here by raising by default and warning if enabling has been forced.
+    enabled = bool(int(os.environ.get(enabled_env_var, "0")))
+    if enabled:
+        return deprecated(message)
+    else:
+
+        def _decorator(cls: Type[Backend]) -> Type[Backend]:
+            def _no_generate(obj) -> Type["StencilObject"]:
+                raise NotImplementedError(
+                    f"Disabled '{cls.name}' backend: 'f{message}'\n",
+                    f"You can still enable the backend by hand using the environment variable '{enabled_env_var}=1'",
+                )
+
+            # Replace generate method with raise
+            if not hasattr(cls, "generate"):
+                raise ValueError(f"Coding error. Expected a generate method on {cls}")
+            # Flag that it got disabled for register lookup
+            cls.disabled = True  # type: ignore
+            cls.generate = _no_generate  # type: ignore
+            return cls
+
+        return _decorator

src/gt4py/cartesian/backend/cuda_backend.py

@@ -15,7 +15,7 @@
 from typing import TYPE_CHECKING, Any, Dict, Optional, Tuple, Type
 
 from gt4py import storage as gt_storage
-from gt4py.cartesian.backend.base import CLIBackendMixin, register
+from gt4py.cartesian.backend.base import CLIBackendMixin, disabled, register
 from gt4py.cartesian.backend.gtc_common import (
     BackendCodegen,
     bindings_main_template,
@@ -125,12 +125,19 @@ def apply_codegen(cls, root, *, module_name="stencil", backend, **kwargs) -> str
         return generated_code
 
 
+@disabled(
+    message="CUDA backend is deprecated. New features developed after February 2024 are not available.",
+    enabled_env_var="GT4PY_GTC_ENABLE_CUDA",
+)
 @register
 class CudaBackend(BaseGTBackend, CLIBackendMixin):
     """CUDA backend using gtc."""
 
     name = "cuda"
-    options = {**BaseGTBackend.GT_BACKEND_OPTS, "device_sync": {"versioning": True, "type": bool}}
+    options = {
+        **BaseGTBackend.GT_BACKEND_OPTS,
+        "device_sync": {"versioning": True, "type": bool},
+    }
     languages = {"computation": "cuda", "bindings": ["python"]}
     storage_info = gt_storage.layout.CUDALayout
     PYEXT_GENERATOR_CLASS = CudaExtGenerator  # type: ignore

src/gt4py/cartesian/backend/gtc_common.py

@@ -278,7 +278,10 @@ def make_extension(
         gt_pyext_sources: Dict[str, Any]
         if not self.builder.options._impl_opts.get("disable-code-generation", False):
             gt_pyext_files = self.make_extension_sources(stencil_ir=stencil_ir)
-            gt_pyext_sources = {**gt_pyext_files["computation"], **gt_pyext_files["bindings"]}
+            gt_pyext_sources = {
+                **gt_pyext_files["computation"],
+                **gt_pyext_files["bindings"],
+            }
         else:
             # Pass NOTHING to the self.builder means try to reuse the source code files
             gt_pyext_files = {}

tests/cartesian_tests/definitions.py

@@ -43,7 +43,7 @@ def _get_backends_with_storage_info(storage_info_kind: str):
     res = []
     for name in _ALL_BACKEND_NAMES:
         backend = gt4pyc.backend.from_name(name)
-        if backend is not None:
+        if not getattr(backend, "disabled", False):
             if backend.storage_info["device"] == storage_info_kind:
                 res.append(_backend_name_as_param(name))
     return res

tests/cartesian_tests/unit_tests/backend_tests/test_backend.py

@@ -48,7 +48,11 @@ def stencil_def(
             out = pa * fa + pb * fb - pc * fc  # type: ignore  # noqa
 
 
-field_info_val = {0: ("out", "fa"), 1: ("out", "fa", "fb"), 2: ("out", "fa", "fb", "fc")}
+field_info_val = {
+    0: ("out", "fa"),
+    1: ("out", "fa", "fb"),
+    2: ("out", "fa", "fb", "fc"),
+}
 parameter_info_val = {0: ("pa",), 1: ("pa", "pb"), 2: ("pa", "pb", "pc")}
 unreferenced_val = {0: ("pb", "fb", "pc", "fc"), 1: ("pc", "fc"), 2: ()}
 
@@ -168,5 +172,23 @@ def test_toolchain_profiling(backend_name: str, mode: int, rebuild: bool):
         assert build_info["load_time"] > 0.0
 
 
+@pytest.mark.parametrize("backend_name", ["cuda"])
+def test_deprecation_gtc_cuda(backend_name: str):
+    # Default deprecation, raise an error
+    build_info: Dict[str, Any] = {}
+    builder = (
+        StencilBuilder(cast(StencilFunc, stencil_def))
+        .with_backend(backend_name)
+        .with_externals({"MODE": 2})
+        .with_options(
+            name=stencil_def.__name__,
+            module=stencil_def.__module__,
+            build_info=build_info,
+        )
+    )
+    with pytest.raises(NotImplementedError):
+        builder.build()
+
+
 if __name__ == "__main__":
     pytest.main([__file__])