Arm backend: Reuse identical CONST_SHAPE nodes (#19770)

Cache CONST_SHAPE nodes created by InsertConstShapesPass and reuse them
when a later view/repeat needs the same shape. This removes duplicate
shape constants.

This improvement is model dependent. Models with few repeated literal
shapes will not see any meaningful change, but some models can benefit
from it notably.

The table below shows the results of a local test lowering DeiT Tiny to
TOSA-FP. The lowering time reduced in this run, likely because passes
following InsertConstShapesPass had fewer nodes to iterate over.

| Metric         | Baseline | Optimized | Delta            |
| -------------- | -------- | --------- | ---------------- |
| Total ops      | 2106     | 1736      | -370 (-17.6%)    |
| CONST_SHAPE    | 466      | 96        | -370 (-79.4%)    |
| TOSA size      | 23.82 MB | 23.75 MB  | -71.6 KB (-0.3%) |
| Execution time | 118.7 s  | 78.4 s    | -40.3 s (-34.0%) |

Signed-off-by: Martin Lindström <Martin.Lindstroem@arm.com>

Author: martinlsm

backends/arm/_passes/insert_const_shapes.py

@@ -26,6 +26,10 @@ class InsertConstShapesPass(ArmPass):
         exir_ops.edge.aten.repeat.default,
     }
 
+    def __init__(self) -> None:
+        super().__init__()
+        self._const_shape_cache: dict[tuple[int, ...], Any] = {}
+
     @staticmethod
     def _is_shape_arg(arg: Any) -> bool:
         """Return True when `arg` looks like a literal shape list/tuple."""
@@ -46,13 +50,17 @@ def call_operator(self, op, args, kwargs, meta, updated: Optional[bool] = False)
                     # Insert a const node for the shape argument
                     if op == exir_ops.edge.aten.view_copy.default:
                         arg = meta.data["val"].shape
-                    const_node = super().call_shape_operator(
-                        exir_ops.backend.tosa.CONST_SHAPE.default,
-                        (arg,),
-                        {},
-                        meta,
-                        True,
-                    )
+                    shape = tuple(arg)
+                    const_node = self._const_shape_cache.get(shape)
+                    if const_node is None:
+                        const_node = super().call_shape_operator(
+                            exir_ops.backend.tosa.CONST_SHAPE.default,
+                            (arg,),
+                            {},
+                            meta,
+                            True,
+                        )
+                        self._const_shape_cache[shape] = const_node
                     new_args.append(const_node)
                     updated = True
                 else: