feat[next][dace]: Build clean nestedSDFG without unused data connecto…

…rs (#1628)

The main objective of this PR is to reduce the GT4Py dependency on DaCe
(especially on DaCe transformations) to produce clean SDFGs. Instead of
relying on the `PruneConnectors` pass to remove unused data connectors
in nested SDFGs, these connectors can be avoided in the first place,
when lowering let-lambdas to nested SDFGs.

src/gt4py/next/program_processors/runners/dace_fieldview/gtir_to_sdfg.py

@@ -19,7 +19,6 @@
 from typing import Any, Dict, List, Optional, Protocol, Sequence, Set, Tuple, Union
 
 import dace
-import dace.transformation.dataflow as dace_dataflow
 
 from gt4py import eve
 from gt4py.eve import concepts
@@ -395,13 +394,21 @@ def visit_Lambda(
         lambda_symbols = self.global_symbols | {
             pname: type_ for pname, (_, type_) in lambda_args_mapping.items()
         }
+        # obtain the set of symbols that are used in the lambda node and all its child nodes
+        used_symbols = {str(sym.id) for sym in eve.walk_values(node).if_isinstance(gtir.SymRef)}
 
         nsdfg = dace.SDFG(f"{sdfg.label}_nested")
         nstate = nsdfg.add_state("lambda")
 
+        # add sdfg storage for the symbols that need to be passed as input parameters,
+        # that is only the symbols that are used in the context of the lambda node
         self._add_sdfg_params(
             nsdfg,
-            [gtir.Sym(id=p_name, type=p_type) for p_name, p_type in lambda_symbols.items()],
+            [
+                gtir.Sym(id=p_name, type=p_type)
+                for p_name, p_type in lambda_symbols.items()
+                if p_name in used_symbols
+            ],
         )
 
         lambda_nodes = GTIRToSDFG(self.offset_provider, lambda_symbols.copy()).visit(
@@ -527,9 +534,5 @@ def build_sdfg_from_gtir(
     sdfg = sdfg_genenerator.visit(program)
     assert isinstance(sdfg, dace.SDFG)
 
-    # nested-SDFGs for let-lambda may contain unused symbols, in which case
-    # we can remove unnecesssary data connectors (not done by dace simplify pass)
-    sdfg.apply_transformations_repeated(dace_dataflow.PruneConnectors)
-
     sdfg.simplify()
     return sdfg