[export] avoid debug name crash for dim hints (pytorch#139104)

Fixes #ISSUE_NUMBER

Pull Request resolved: pytorch#139104
Approved by: https://github.com/ezyang

test/export/test_export.py

@@ -3128,6 +3128,21 @@ def forward(self, image, crop_height, crop_width):
         args = (torch.rand(3, 700, 700), 150, 150)
         self.assertEqual(ecrop.module()(*args), ecrop(*args))
 
+    def test_dim_dynamic_divisibility(self):
+        class M(torch.nn.Module):
+            def forward(self, x):
+                if x.size(0) % 2 == 0:
+                    return x.clone() * 2
+                else:
+                    return x.clone() * 0
+
+        input1 = (torch.randn(4),)
+        model = M()
+        dynamic_shapes = {
+            "x": {0: torch.export.Dim.DYNAMIC},
+        }
+        export(model, input1, dynamic_shapes=dynamic_shapes)
+
     def test_export_func_with_kwargs(self):
         class Module(torch.nn.Module):
             def forward(self, arg1, arg2, kw1, kw2):

torch/fx/experimental/symbolic_shapes.py

@@ -2436,7 +2436,12 @@ def solve(self) -> None:
                 ):
                     if self._is_supported_congruence(congruence):
                         base, divisor = congruence.args
-                        tmp_name = f"_{self._dcp.source_name_to_debug_name[self._dcp.symbol_to_source[s][0].name()]}"
+                        tmp_name = "_" + str(
+                            self._dcp.source_name_to_debug_name.get(
+                                self._dcp.symbol_to_source[s][0].name(),
+                                self._dcp.symbol_to_source[s][0].name(),
+                            )
+                        )
                         tmp = sympy.Symbol(tmp_name, integer=True)
                         from torch._dynamo.source import ConstantSource