Avoid canonicalization of slices when merging non-overlapping slices in `local_subtensor_merge`

pytensor/tensor/rewriting/subtensor.py

@@ -370,9 +370,10 @@ def local_subtensor_merge(fgraph, node):
     """
     from pytensor.scan.op import Scan
 
-    if isinstance(node.op, Subtensor):
     u = node.inputs[0]
-        if u.owner and isinstance(u.owner.op, Subtensor):
+    if not (u.owner is not None and isinstance(u.owner.op, Subtensor)):
+        return None
+
     # We can merge :)
     # x actual tensor on which we are picking slices
     x = u.owner.inputs[0]
@@ -425,9 +426,7 @@ def local_subtensor_merge(fgraph, node):
     merged_slices = tuple(as_index_constant(s) for s in merged_slices)
     subtens = Subtensor(merged_slices)
 
-            sl_ins = get_slice_elements(
-                merged_slices, lambda x: isinstance(x, Variable)
-            )
+    sl_ins = get_slice_elements(merged_slices, lambda x: isinstance(x, Variable))
     # Do not call make_node for test_value
     out = subtens(x, *sl_ins)
 
@@ -826,6 +825,12 @@ def merge_two_slices(fgraph, slice1, len1, slice2, len2):
     if not isinstance(slice1, slice):
         raise ValueError("slice1 should be of type `slice`")
 
+    # Simple case where one of the slices is useless
+    if is_full_slice(slice1):
+        return slice2
+    elif is_full_slice(slice2):
+        return slice1
+
     sl1, reverse1 = get_canonical_form_slice(slice1, len1)
     sl2, reverse2 = get_canonical_form_slice(slice2, len2)