Cleanup Scan symbolic buffer size graph

Graph was being broken by Scalar/Tensor conversions that prevented fusion

Author: ricardoV94

pytensor/tensor/subtensor.py

@@ -33,7 +33,9 @@
     alloc,
     get_scalar_constant_value,
     nonzero,
-    scalar_from_tensor,
+)
+from pytensor.tensor.basic import (
+    constant as tensor_constant,
 )
 from pytensor.tensor.blockwise import vectorize_node_fallback
 from pytensor.tensor.elemwise import DimShuffle
@@ -296,13 +298,30 @@ def get_canonical_form_slice(
     """
     from pytensor.tensor import ge, lt, sign, switch
 
+    def undo_scalarization(x):
+        """Undo scalarization of a variable.
+
+        PyTensor Basic index operations use ScalarVariables for the indices/slice arguments.
+        But reasoning symbolically about the result of multiple indexing operations, we usually
+        want to work on TensorVariables, since rewrites work on those and not ScalarVariables.
+
+        This function undoes ScalarFromTensor operation or converts ScalarConstants to TensorConstants.
+        """
+        if isinstance(x, ScalarVariable):
+            if isinstance(x, ScalarConstant):
+                return tensor_constant(x.data, dtype=x.dtype)
+            elif x.owner is not None and isinstance(x.owner.op, ScalarFromTensor):
+                return x.owner.inputs[0]
+        return x
+
     # Other non-slice types are the scalar indexing case
     if not isinstance(theslice, slice):
+        theslice = undo_scalarization(theslice)
         if isinstance(theslice, int | np.integer | ScalarVariable) or (
             isinstance(theslice, TensorVariable) and theslice.ndim == 0
         ):
             cano = switch(lt(theslice, 0), (theslice + length), theslice)
-            return scalar_from_tensor(cano), 1
+            return cano, 1
         raise ValueError(f"Slice {theslice} is not a supported slice type.")
 
     # At this point we have a slice object. Possibly with symbolic inputs.
@@ -312,7 +331,7 @@ def analyze(x):
             x_constant = as_index_literal(x)
             is_constant = True
         except NotScalarConstantError:
-            x_constant = x
+            x_constant = undo_scalarization(x)
             is_constant = False
         return x_constant, is_constant