[dynamic shapes] make backed_size_oblivious behavior consistent b/w symbolic_shapes/inductor (#164796)

Summary: call guard_or_ directly to enable backed_size_obl in inductor calls to guard_or

Test Plan: CI and unit test added.

Differential Revision: D84009392

Pull Request resolved: https://github.com/pytorch/pytorch/pull/164796
Approved by: https://github.com/laithsakka

test/test_dynamic_shapes.py

@@ -1426,6 +1426,15 @@ class f(torch.nn.Module):
                 f(torch.tensor([1]), torch.tensor([1])), torch.tensor([20])
             )
 
+    @fresh_cache()
+    def test_slice_backed_size_oblivious(self):
+        @torch.compile(backend="inductor", fullgraph=True, dynamic=True)
+        def f(x):
+            return x[:5]
+
+        with torch.fx.experimental._config.patch(backed_size_oblivious=True):
+            f(torch.randn(10, 10))
+
     def test_baddbmm_symint(self):
         from torch._subclasses.fake_tensor import FakeTensorMode
 

torch/_inductor/sizevars.py

@@ -453,9 +453,23 @@ class SizeVarAllocator:
     # Similar to the functions guard_or_false/guard_or_true in symbolic_shapes.py
     # but operates on sympy expressions instead of symnodes. see Note [guard_or_].
     def guard_or_false(self, left):
+        import torch.fx.experimental._config as exp_config
+
+        if exp_config.backed_size_oblivious:
+            static_val = self.shape_env._maybe_evaluate_static(left)
+            if static_val is not None:
+                return static_val
+            return False
         return self.evaluate_expr(left, fallback_value=False)
 
     def guard_or_true(self, left):
+        import torch.fx.experimental._config as exp_config
+
+        if exp_config.backed_size_oblivious:
+            static_val = self.shape_env._maybe_evaluate_static(left)
+            if static_val is not None:
+                return static_val
+            return True
         return self.evaluate_expr(left, fallback_value=True)
 
     # The evaluate functions evaluate some symbolic sympy expression