refactor[next]: remove FencilDefinition definition (#1774)

After this commit `FencilDefinition`s are completely removed. Next step
could be to cleanup `itir` -> `gtir` everywhere.

docs/user/next/advanced/HackTheToolchain.md

@@ -15,7 +15,7 @@ from gt4py import eve
 
 ```python
 cached_lowering_toolchain = gtx.backend.DEFAULT_TRANSFORMS.replace(
-    past_to_itir=gtx.ffront.past_to_itir.past_to_itir_factory(cached=False)
+    past_to_itir=gtx.ffront.past_to_itir.past_to_gtir_factory(cached=False)
 )
 ```
 

src/gt4py/next/backend.py

@@ -16,7 +16,6 @@
 from gt4py.next import allocators as next_allocators
 from gt4py.next.ffront import (
     foast_to_gtir,
-    foast_to_itir,
     foast_to_past,
     func_to_foast,
     func_to_past,
@@ -41,7 +40,7 @@
 
 ARGS: typing.TypeAlias = arguments.JITArgs
 CARG: typing.TypeAlias = arguments.CompileTimeArgs
-IT_PRG: typing.TypeAlias = itir.FencilDefinition | itir.Program
+IT_PRG: typing.TypeAlias = itir.Program
 
 
 INPUT_DATA: typing.TypeAlias = DSL_FOP | FOP | DSL_PRG | PRG | IT_PRG
@@ -93,7 +92,7 @@ class Transforms(workflow.MultiWorkflow[INPUT_PAIR, stages.CompilableProgram]):
     )
 
     past_to_itir: workflow.Workflow[AOT_PRG, stages.CompilableProgram] = dataclasses.field(
-        default_factory=past_to_itir.past_to_itir_factory
+        default_factory=past_to_itir.past_to_gtir_factory
     )
 
     def step_order(self, inp: INPUT_PAIR) -> list[str]:
@@ -126,7 +125,7 @@ def step_order(self, inp: INPUT_PAIR) -> list[str]:
                 )
             case PRG():
                 steps.extend(["past_lint", "field_view_prog_args_transform", "past_to_itir"])
-            case itir.FencilDefinition() | itir.Program():
+            case itir.Program():
                 pass
             case _:
                 raise ValueError("Unexpected input.")
@@ -135,17 +134,6 @@ def step_order(self, inp: INPUT_PAIR) -> list[str]:
 
 DEFAULT_TRANSFORMS: Transforms = Transforms()
 
-# FIXME[#1582](havogt): remove after refactoring to GTIR
-# note: this step is deliberately placed here, such that the cache is shared
-_foast_to_itir_step = foast_to_itir.adapted_foast_to_itir_factory(cached=True)
-LEGACY_TRANSFORMS: Transforms = Transforms(
-    past_to_itir=past_to_itir.past_to_itir_factory(to_gtir=False),
-    foast_to_itir=_foast_to_itir_step,
-    field_view_op_to_prog=foast_to_past.operator_to_program_factory(
-        foast_to_itir_step=_foast_to_itir_step
-    ),
-)
-
 
 # TODO(tehrengruber): Rename class and `executor` & `transforms` attribute. Maybe:
 #  `Backend` -> `Toolchain`

src/gt4py/next/ffront/decorator.py

@@ -34,7 +34,6 @@
 from gt4py.next.ffront import (
     field_operator_ast as foast,
     foast_to_gtir,
-    foast_to_itir,
     past_process_args,
     signature,
     stages as ffront_stages,
@@ -186,7 +185,7 @@ def _all_closure_vars(self) -> dict[str, Any]:
         return transform_utils._get_closure_vars_recursively(self.past_stage.closure_vars)
 
     @functools.cached_property
-    def itir(self) -> itir.FencilDefinition:
+    def gtir(self) -> itir.Program:
         no_args_past = toolchain.CompilableProgram(
             data=ffront_stages.PastProgramDefinition(
                 past_node=self.past_stage.past_node,
@@ -561,7 +560,7 @@ def with_grid_type(self, grid_type: common.GridType) -> FieldOperator:
     #  a different backend than the one of the program that calls this field operator. Just use
     #  the hard-coded lowering until this is cleaned up.
     def __gt_itir__(self) -> itir.FunctionDefinition:
-        return foast_to_itir.foast_to_itir(self.foast_stage)
+        return foast_to_gtir.foast_to_gtir(self.foast_stage)
 
     # FIXME[#1582](tehrengruber): remove after refactoring to GTIR
     def __gt_gtir__(self) -> itir.FunctionDefinition: