fix torch backend batchnorm backward (#17201)

extra/torch_backend/backend.py

@@ -779,21 +779,17 @@ def native_batch_norm(input, weight, bias, running_mean, running_var, training,
 @torch.library.impl("aten::native_batch_norm_backward", "privateuseone")
 def native_batch_norm_backward(grad_out, input, weight, running_mean, running_var, save_mean, save_invstd, train, eps, output_mask):
   grad_out_t, input_t = unwrap(grad_out), unwrap(input)
-  weight_t = unwrap(weight) if weight is not None else None
-  save_mean_t = unwrap(save_mean)
-  save_invstd_t = unwrap(save_invstd)
-  out = input_t.batchnorm(weight_t, None, save_mean_t, save_invstd_t)
-  targets = [t for t, m in zip([input_t, weight_t], output_mask[:2]) if t is not None and m]
-  if targets:
-    grads = out.gradient(*targets, gradient=grad_out_t)
-    grad_input = grads.pop(0) if output_mask[0] else None
-    grad_weight = grads.pop(0) if output_mask[1] and weight_t is not None else None
-  else:
-    grad_input, grad_weight = None, None
-  grad_bias = grad_out_t.sum(axis=tuple(x for x in range(grad_out_t.ndim) if x != 1)) if output_mask[2] else None
-  return (wrap(grad_input) if grad_input is not None else None,
-          wrap(grad_weight) if grad_weight is not None else None,
-          wrap(grad_bias) if grad_bias is not None else None)
+  dims, shape = tuple(x for x in range(input_t.ndim) if x != 1), (1, -1) + (1,)*(input_t.ndim-2)
+  # training differentiates the batch stats it was given, eval treats the running stats as constants
+  if train: mean, invstd = unwrap(save_mean), unwrap(save_invstd)
+  else: mean, invstd = unwrap(running_mean), unwrap(running_var).add(eps).rsqrt()
+  xhat = (input_t - mean.reshape(shape)) * invstd.reshape(shape)
+  grad_bias, grad_weight = grad_out_t.sum(axis=dims), (grad_out_t * xhat).sum(axis=dims)
+  grad_input = grad_out_t if not train else \
+    grad_out_t - (grad_bias.reshape(shape) + xhat * grad_weight.reshape(shape)) / (input_t.numel() // input_t.shape[1])
+  grad_input = grad_input * invstd.reshape(shape) * (unwrap(weight).reshape(shape) if weight is not None else 1)
+  return (wrap(grad_input) if output_mask[0] else None, wrap(grad_weight) if output_mask[1] else None,
+          wrap(grad_bias) if output_mask[2] else None)
 
 # _pad_circular is not CompositeImplicitAutograd (unlike reflect/replicate pad)
 # we need torch.autograd.Function with explicit AutogradPrivateUse1 registration

extra/torch_backend/test.py

@@ -343,6 +343,21 @@ class TestTorchBackend(unittest.TestCase):
     assert b.shape == (4, 2, 3)
     np.testing.assert_equal(b.cpu().numpy(), a.cpu().numpy().transpose(2, 0, 1))
 
+  def test_batchnorm_backward_realized_stats(self):
+    # the saved stats are a function of input in training, so grad_input must flow through them even when handed in realized.
+    # the backward eps is unused in training: torch differentiates the save_invstd it was given
+    x0, g0 = torch.randn(8, 4, 3, 3), torch.randn(8, 4, 3, 3)
+    def run(dev, bwd_eps):
+      x, go = x0.to(dev), g0.to(dev)
+      w, b = torch.linspace(0.5, 2.0, 4).to(dev), torch.zeros(4, device=dev)
+      rm, rv = torch.zeros(4, device=dev), torch.ones(4, device=dev)
+      out, sm, si = torch.ops.aten.native_batch_norm(x, w, b, rm, rv, True, 0.1, 1e-5)
+      grads = torch.ops.aten.native_batch_norm_backward(go, x, w, rm, rv, sm.clone().detach(), si.clone().detach(),
+                                                        True, bwd_eps, [True,True,True])
+      return [t.cpu().numpy() for t in grads]
+    for bwd_eps in [1e-5, 0.3]:
+      for got, want in zip(run(device, bwd_eps), run("cpu", bwd_eps)): np.testing.assert_allclose(got, want, atol=1e-4, rtol=1e-3)
+
   def test_batchnorm_unsqueeze(self):
     bn = torch.nn.BatchNorm2d(4).to(device)
     x = torch.randn(8, 4, 3, 3, device=device)
@@ -773,6 +788,12 @@ class TestBackendHelpers(unittest.TestCase):
     # unwrap casts to the tiny impl, so a tensor from another backend must be refused rather than reinterpreted
     with self.assertRaises(RuntimeError): extra.torch_backend.backend.unwrap(torch.ones(4))
 
+  def test_update_metadata_rejects_foreign_tensor(self):
+    # resizing a tensor we don't own would expose memory past its allocation
+    t = torch.ones(4)
+    with self.assertRaises(RuntimeError): extra.torch_backend.backend.mod.update_metadata(t, [8], [1], 0)
+    self.assertEqual(t.shape, (4,))
+
   def test_unwrap_parameter_and_detached(self):
     # nn.Parameter and detach rebuild the base OpaqueTensorImpl, which unwrap still has to accept
     extra.torch_backend.backend.unwrap(torch.nn.Parameter(torch.ones(4, device="tiny")))

extra/torch_backend/wrapped_tensor.cpp

@@ -124,18 +124,21 @@ at::Tensor wrap_tensor(py::object &py_obj, c10::ScalarType dtype, c10::DeviceInd
     sizes, strides, storage_offset);
 }
 
+// shallow_copy_and_detach (nn.Parameter, aten.detach) rebuilds the base OpaqueTensorImpl, so that is the type every tiny tensor has
+at::OpaqueTensorImpl<std::shared_ptr<c10::SafePyObject>> *tiny_impl(const at::Tensor &tensor) {
+  auto* impl = dynamic_cast<at::OpaqueTensorImpl<std::shared_ptr<c10::SafePyObject>>*>(tensor.unsafeGetTensorImpl());
+  TORCH_CHECK(impl != nullptr, "expected a tiny tensor, got a ", tensor.device().str(), " one. move it with .to(\"tiny\") first");
+  return impl;
+}
+
 py::object unwrap_tensor(const at::Tensor &tensor) {
-  auto* impl = tensor.unsafeGetTensorImpl();
-  // shallow_copy_and_detach (nn.Parameter, aten.detach) rebuilds the base OpaqueTensorImpl, so that is the type every tiny tensor has
-  auto* opaque_impl = dynamic_cast<at::OpaqueTensorImpl<std::shared_ptr<c10::SafePyObject>>*>(impl);
-  TORCH_CHECK(opaque_impl != nullptr, "expected a tiny tensor, got a ", tensor.device().str(), " one. move it with .to(\"tiny\") first");
-  std::shared_ptr<c10::SafePyObject> tiny = opaque_impl->opaque_handle();
+  std::shared_ptr<c10::SafePyObject> tiny = tiny_impl(tensor)->opaque_handle();
   return py::reinterpret_borrow<py::object>(tiny->ptr(getPyInterpreter()));
 }
 
 void update_metadata(const at::Tensor &tensor, const std::vector<int64_t> &sizes,
                      const std::vector<int64_t> &strides, int64_t storage_offset) {
-  auto* impl = tensor.unsafeGetTensorImpl();
+  auto* impl = tiny_impl(tensor);
   impl->set_allow_tensor_metadata_change(true);
   impl->set_sizes_and_strides(sizes, strides, storage_offset);
 }