Make SymFloat behave symmetrically with float in torch.tensor (#109513)

Previously, SymFloat would force double precision.  That's wrong;
instead, we must respect default dtype.

Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: #109513
Approved by: https://github.com/voznesenskym

test/test_proxy_tensor.py

@@ -1047,6 +1047,23 @@ def forward(self, a_1):
     mul = torch.ops.aten.mul.Tensor(a_1, _local_scalar_dense);  a_1 = _local_scalar_dense = None
     return mul""")
 
+    def test_tensor_symfloat(self):
+        def f(a):
+            r = torch.tensor(a.size(0) ** 2.0)
+            assert r.dtype is torch.float
+            return r
+
+        gm = make_fx(f, tracing_mode="symbolic")(torch.randn(2))
+        r = str(gm.code).strip()
+        # NB: this specializes, which is fine, the point is to make sure the
+        # dtype inference is correct
+        self.assertExpectedInline(r, """\
+def forward(self, a_1):
+    _tensor_constant0 = self._tensor_constant0
+    lift_fresh_copy = torch.ops.aten.lift_fresh_copy.default(_tensor_constant0);  _tensor_constant0 = None
+    return lift_fresh_copy""")
+        self.assertEqual(gm._tensor_constant0, torch.tensor(4.0))
+
     def test_item_to_constructor(self):
         def f(a):
             r = a.item()

torch/csrc/utils/tensor_new.cpp

@@ -123,7 +123,7 @@ ScalarType infer_scalar_type(PyObject* obj) {
     return ScalarType::Long;
   }
   if (torch::is_symfloat(obj)) {
-    return ScalarType::Double;
+    return torch::tensors::get_default_scalar_type();
   }
 #ifdef USE_NUMPY
   if (is_numpy_available()) {
@@ -211,7 +211,15 @@ void recursive_store(
     if (is_symfloat) {
       auto new_obj = py::reinterpret_borrow<py::object>(obj);
       auto val = new_obj.cast<c10::SymFloat>();
-      *(double*)data = val.guard_float(__FILE__, __LINE__);
+      const double double_val = val.guard_float(__FILE__, __LINE__);
+      switch (elementSize) {
+        case 8:
+          *reinterpret_cast<double*>(data) = double_val;
+          break;
+        case 4:
+          *reinterpret_cast<float*>(data) = static_cast<float>(double_val);
+          break;
+      }
       return;
     }
     if (is_symint) {