Fix lerp weight type promotion (#141117)

Fixes #140601

Enable `promote_inputs_to_common_dtype` when tensors not same dtype when invoke `lerp` function.

For `lerp_Tensor`
- Check whether same `dtype` of tensors, enable promote if not
- Remove type check assert

For `lerp_Scalar`
- Seems already enable `promote_inputs_to_common_dtype` by default, just remove the type check. Make sure promote behavior consistent with `lerp_Tensor`

`lerp_Scalar` get TensorIteratorConfig from here
c37185c76a/aten/src/ATen/TensorIterator.cpp (L979-L985)

**Test Result**
Test case in issue passed

```python
>>> import torch
>>>
>>> x = torch.ones(2, 2, dtype=torch.float64)
>>> w = torch.ones(2, 2, dtype=torch.float64)
>>> s = torch.tensor(2.2)
>>> x.lerp_(w, s)
tensor([[1., 1.],
        [1., 1.]], dtype=torch.float64)

>>> x = torch.ones(2, 2, dtype=torch.float16)
>>> w = torch.ones(2, 2, dtype=torch.float16)
>>> s = torch.tensor(2.2)
>>> x.lerp_(w, s)
tensor([[1., 1.],
        [1., 1.]], dtype=torch.float16)

```

```bash
$ pytest test/test_binary_ufuncs.py -k 'test_lerp_tensor_type_promotion or test_lerp_scalar_type_promotion'
```
![image](https://github.com/user-attachments/assets/288a5294-a9ee-47f3-bbf7-d4ff986f3ba8)

```bash
$ lintrunner
```
![image](https://github.com/user-attachments/assets/d469836f-5c49-4d89-a2fd-379cad4db3af)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/141117
Approved by: https://github.com/janeyx99

Co-authored-by: Jane (Yuan) Xu <31798555+janeyx99@users.noreply.github.com>

aten/src/ATen/native/Lerp.cpp

@@ -16,10 +16,16 @@ TORCH_META_FUNC(lerp_Tensor)(
     const Tensor& self, const Tensor& end, const Tensor& weight) {
   TORCH_CHECK(self.dtype() == end.dtype(), "expected dtype ", self.dtype(),
               " for `end` but got dtype ", end.dtype());
+  bool promote_weight = weight.dim() == 0;
+  if (!promote_weight) {
     TORCH_CHECK(self.dtype() == weight.dtype(), "expected dtype ", self.dtype(),
                 " for `weight` but got dtype ", weight.dtype());
+  }
   build(at::TensorIteratorConfig()
         .allow_cpu_scalars(true)
+        .promote_inputs_to_common_dtype(promote_weight)
+        .enforce_safe_casting_to_output(promote_weight)
+        .cast_common_dtype_to_outputs(promote_weight)
         .add_output(maybe_get_output())
         .add_const_input(self)
         .add_const_input(end)

test/test_binary_ufuncs.py

@@ -3519,6 +3519,24 @@ class TestBinaryUfuncs(TestCase):
                 expected = torch.lerp(xref, yref, wref).to(dtype)
                 self.assertEqual(actual, expected, atol=0.0, rtol=0.0)
 
+    @dtypes(torch.float, torch.double, torch.cfloat, torch.cdouble)
+    def test_lerp_weight_scalar_tensor_promotion(self, device, dtype):
+        start = make_tensor((5, 5), dtype=dtype, device=device, low=1, high=100)
+        end = make_tensor((5, 5), dtype=dtype, device=device, low=1, high=100)
+        weight = torch.rand((), dtype=torch.float, device=device)
+
+        actual = torch.lerp(start, end, weight)
+        expected = start + weight.to(dtype) * (end - start)
+        self.assertEqual(expected, actual)
+
+    @dtypes(torch.double, torch.cfloat, torch.cdouble)
+    def test_lerp_weight_tensor_promotion_error(self, device, dtype):
+        start = make_tensor((5, 5), dtype=dtype, device=device, low=1, high=100)
+        end = make_tensor((5, 5), dtype=dtype, device=device, low=1, high=100)
+        weight = torch.rand((5, 5), dtype=torch.float, device=device)
+        with self.assertRaisesRegex(RuntimeError, "expected dtype"):
+            torch.lerp(start, end, weight)
+
     def _test_logaddexp(self, device, dtype, base2):
         if base2:
             ref_func = np.logaddexp2

torch/_meta_registrations.py

@@ -6972,6 +6972,7 @@ def lerp(start, end, weight):
     )
     args = [start, end]
     if isinstance(weight, TensorLike):
+        if weight.ndim != 0:
             torch._check(
                 start.dtype == weight.dtype,
                 lambda: f"expected dtype {start.dtype} for `weight`, but got dtype {weight.dtype}",