Simplify Subtensor.infer_shape for reversed slices

Author: ricardoV94

pytensor/tensor/subtensor.py

@@ -948,6 +948,9 @@ def perform(self, node, inputs, out_):
         out[0] = np.asarray(x.__getitem__(cdata))
 
     def infer_shape(self, fgraph, node, shapes):
+        def _is_constant(const, x):
+            return isinstance(const, Constant) and const.data.item() == x
+
         xshp = shapes[0]
         assert len(xshp) == node.inputs[0].ndim
         outshp = []
@@ -961,10 +964,17 @@ def infer_shape(self, fgraph, node, shapes):
                 # If it is the default (None, None, None) slice, or a variant,
                 # the shape will be xl
                 if (
-                    (idx.start in [None, 0])
-                    and (idx.stop in [None, sys.maxsize])
-                    and (idx.step is None or idx.step == 1)
+                    (idx.start is None or _is_constant(idx.start, 0))
+                    and (idx.stop is None or _is_constant(idx.stop, sys.maxsize))
+                    and (idx.step is None or _is_constant(idx.step, 1))
+                ):
+                    outshp.append(xl)
+                elif (
+                    (idx.start is None)
+                    and (idx.stop is None)
+                    and _is_constant(idx.step, -1)
                 ):
+                    # Reverse slice
                     outshp.append(xl)
                 else:
                     cnf = get_canonical_form_slice(idx, xl)[0]