feat(dynamo): IS#160752 make F.one_hot work with jacfwd + torch.compile(dynamic=True) (#160837)

Fixes #160752

# Background:
`torch.func.jacfwd` is implemented as vmap over forward-mode JVP. With torch.compile(dynamic=True), FakeTensor + SymInt shape reasoning is used while tracing through the transform. The old vmap rule for one_hot decomposed into “zeros_symint + scatter,” which interacted poorly with the transform stack and dynamic shapes, leading to failures mid-trace. Using a functional equality construction makes one_hot composable with vmap/JVP and friendly to dynamic shape tracing.

# Changes:
- functorch vmap batching rule for `aten::one_hot` now uses a purely functional formulation:
- Replace “zeros + scatter” with eq(self.unsqueeze(-1), arange(num_classes)).to(kLong) under FuncTorchBatched.
- one_hot native path remains unchanged for regular eager; vmap transform no longer relies on scatter, which was fragile under dynamic shape tracing.

The minimal repro from the issue is now fixed:
```python
import torch
import torch.nn.functional as F

MAX, BATCH = 3, 37

def func(x, idxs):
    return x.square() * F.one_hot(idxs, MAX)

def jacfunc(x, idxs):
    return torch.func.jacfwd(func, argnums=0)(x, idxs)

idxs = torch.randint(MAX, (BATCH,), dtype=torch.int64)
x = torch.rand((BATCH, MAX), dtype=torch.float64)

# eager
out_eager = jacfunc(x, idxs)

# compiled dynamic
jacfunc_c = torch.compile(jacfunc, dynamic=True)
out_comp = jacfunc_c(x, idxs)

torch.testing.assert_close(out_eager, out_comp)
```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/160837
Approved by: https://github.com/guilhermeleobas, https://github.com/zou3519

aten/src/ATen/functorch/BatchRulesModules.cpp

@@ -213,40 +213,22 @@ static cudnn_grid_sample_backward_batch_rule(
   return grid_sample_backward_helper_out(std::move(bw_out), 0, 0, bdim_size);
 }
 
-// TODO: replace with targetable functionalization
+// uses functional formulation for one_hot under vmap to be compatible with
+// fakeTensor/dynamic shapes and compiled functorch transforms.
+// mirrors the meta path in aten/src/ATen/native/Onehot.cpp,
+// but requires explicit positive num_classes under vmap to avoid
+// data-dependent output shapes.
 static Tensor one_hot_decomposition_hack(const Tensor &self, int64_t num_classes) {
     TORCH_CHECK(self.dtype() == kLong, "one_hot is only applicable to index tensor.");
-    auto shape = self.sym_sizes().vec();
-
-    // empty tensor could be converted to one hot representation,
-    // but shape inference is not possible.
-    if (self.sym_numel() == 0) {
-        if (num_classes <= 0) {
-            TORCH_CHECK(false, "Can not infer total number of classes from empty tensor.");
-        } else {
-            shape.emplace_back(num_classes);
-            return at::empty_symint(shape, self.options());
-        }
-    }
 
+    // disallow implicit inference under vmap; this would be data-dependent
+    // and is intentionally guarded by Dynamo in torch/_dynamo/variables/torch.py.
     TORCH_CHECK(num_classes > 0, "When vmap-ing torch.nn.functional.one_hot, please "
         "provide an explicit positive num_classes argument.");
 
-    // Disabling all of the following checks. This is OK because scatter has checks too.
-    // Maybe one_hot should be a primitive wrt autograd so we don't have to deal with this.
-    // // non-empty tensor
-    // if (self.device().type() != at::kCUDA) {
-    //   //for cuda, rely on device assert thrown by scatter
-    //   TORCH_CHECK(self.min().item().toLong() >= 0, "Class values must be non-negative.");
-    // }
-    // if (self.device().type() != at::kCUDA) {
-    //   //rely on device asserts from scatter to avoid sync here
-    //   TORCH_CHECK(num_classes > self.max().item().toLong(), "Class values must be smaller than num_classes.");
-    // }
-
-    shape.emplace_back(num_classes);
-    Tensor ret = at::zeros_symint(shape, self.options());
-    return ret.scatter(-1, self.unsqueeze(-1), 1);
+    const auto options = self.options();
+    at::Tensor index = at::arange(num_classes, options);
+    return at::eq(self.unsqueeze(-1), index).to(at::kLong);
 }
 
 template <typename A, A a, typename C>

aten/src/ATen/native/Onehot.cpp

@@ -34,16 +34,16 @@ Tensor one_hot(const Tensor &self, int64_t num_classes) {
         }
     }
 
-    auto shape = self.sizes().vec();
+    auto shape = self.sym_sizes().vec();
 
     // empty tensor could be converted to one hot representation,
     // but shape inference is not possible.
-    if (self.numel() == 0) {
+    if (self.sym_numel() == 0) {
         if (num_classes <= 0) {
             TORCH_CHECK(false, "Can not infer total number of classes from empty tensor.");
         } else {
-            shape.push_back(num_classes);
-            return at::empty(shape, self.options());
+            shape.emplace_back(num_classes);
+            return at::empty_symint(shape, self.options());
         }
     }
 
@@ -66,8 +66,8 @@ Tensor one_hot(const Tensor &self, int64_t num_classes) {
         }
     }
 
-    shape.push_back(num_classes);
-    Tensor ret = at::zeros(shape, self.options());
+    shape.emplace_back(num_classes);
+    Tensor ret = at::zeros_symint(shape, self.options());
     ret.scatter_(-1, self.unsqueeze(-1), 1);
     return ret;
 }