Repository: pytorch/functorch (language: python)

Issue:
Prelu batching rule (forward + backward)
Adds the batching rule for prelu. Will need to wait on merging of [prelu opinfo fix](https://github.com/pytorch/pytorch/pull/74467) in order to get rid of the last commit that does a hacky fix so that the weight is passed as a positional arg and is therefore batchable


Discussion / hints:

Fix patch:
diff --git a/functorch/csrc/BatchRulesActivation.cpp b/functorch/csrc/BatchRulesActivation.cpp
new file mode 100644
index 000000000..48a87820c
--- /dev/null
+++ b/functorch/csrc/BatchRulesActivation.cpp
@@ -0,0 +1,181 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree.
+
+#include <functorch/csrc/BatchRulesHelper.h>
+#include <functorch/csrc/PlumbingHelper.h>
+#include <ATen/Operators.h>
+
+// NB: most activation functions fit pointwise unary or binary rules.
+// These are only the ones that have special batch rules to help with organization
+namespace at { namespace functorch {
+std::tuple<Tensor,optional<int64_t>> prelu_batch_rule(
+    const Tensor& input, optional<int64_t> input_bdim,
+    const Tensor& weight, optional<int64_t> weight_bdim) {
+  if (!weight_bdim && weight.dim() == 0) {
+    return std::make_tuple(at::prelu(input, weight), input_bdim);
+  }
+
+  const auto input_ = moveBatchDimToFront(input, input_bdim);
+  auto weight_flatten = moveBatchDimToFront(weight, weight_bdim);
+
+  if (weight_flatten.dim() > 1) {
+    // for an input [N, C, ...]
+    // weight can be a non-vector but the total number of elements must be the same as C
+    weight_flatten = at::flatten(weight_flatten, weight_bdim.has_value() ? 1 : 0, -1);
+  }
+
+  const int64_t input_logical_rank = rankWithoutBatchDim(input, input_bdim);
+  VmapDimVector new_shape(weight_flatten.sizes().begin(), weight_flatten.sizes().end());
+  const int64_t final_size = weight_bdim ? (input_logical_rank + 1) : input_logical_rank;
+  new_shape.reserve(final_size);
+
+  if (weight_flatten.dim() == 2 || !weight_bdim) {
+    // if weight (without batching) is not a scalar, its size must match the "channel dimension" of input. To do the
+    // decomposition, we pad the weight to
+
+    // copies checks from prelu if the weight (without vmap) is not a scalar
+    TORCH_CHECK(input_logical_rank > 0, "Not allow zero-dim input tensor.");
+
+    int64_t channel_size = 1; // channel_size default to 1
+    if (input_logical_rank > 1) {
+      const auto channel_dim = input_bdim ? 2 : 1;
+      channel_size = input_.size(channel_dim);
+    }
+    const auto weight_num = weight_flatten.size(-1);
+    TORCH_CHECK(channel_size == weight_num,
+      "Mismatch of parameter numbers and input channel size. Found parameter numbers = ", weight_num,
+      " and channel size = ", channel_size, ".");
+
+    // pads to the left so that the flattened shape matches up with the channel
+    if (!weight_bdim) {
+      new_shape.insert(new_shape.begin(), 1); 
+    } else {
+      new_shape.insert(new_shape.begin() + 1, 1);
+    }
+  }
+
+  for (int64_t i = new_shape.size(); i < final_size; i ++) {
+    new_shape.push_back(1);
+  }
+  TORCH_INTERNAL_ASSERT(new_shape.size() == final_size);
+  const auto weight_padded = weight_flatten.view(new_shape);
+  auto zero_tensor = at::zeros(1, input.options());
+
+  // decomposes function, 
+  auto res = at::maximum(zero_tensor, input_) + weight_padded * at::minimum(zero_tensor, input_);
+  return std::make_tuple(res, 0);
+}
+
+VmapDimVector ensure_shape_with_bdim(const Tensor& input, const bool has_bdim, const int64_t batch_size) {
+  // helper function that get the size of input, ensuring that there's batch dim, without expanding input
+  if (has_bdim) {
+    // sad to have to copy but got garbage if tried to return an IntArrayRef and just do input.sizes()
+    VmapDimVector new_shape(input.sizes().begin(), input.sizes().end());
+    return new_shape;
+  }
+  VmapDimVector new_shape(1, batch_size);
+  new_shape.reserve(input.dim() + 1);
+  new_shape.insert(new_shape.end(), input.sizes().begin(), input.sizes().end());
+  return new_shape;
+}
+
+VmapDimVector shape_maybe_with_bdim(const Tensor& input, const bool need_bdim, const bool has_bdim, const int64_t batch_size) {
+  // if need_bdim, will return the input with a guaranteed bdim. If not, will return the input logical size (no batch dim)
+  if (need_bdim) {
+    return ensure_shape_with_bdim(input, has_bdim, batch_size);
+  } else if (has_bdim) { // !need_bdim && has_bdim
+    VmapDimVector new_shape(input.sizes().begin() + 1, input.sizes().end());
+    return new_shape;
+  } else { // !need_bdim && !has_bdim
+    VmapDimVector new_shape(input.sizes().begin(), input.sizes().end());
+    return new_shape;
+  }
+}
+
+std::tuple<Tensor, Tensor> prelu_backward_batched(
+    const Tensor& grad_out, const Tensor& self, const Tensor& weight,
+    const VmapDimVector& self_grad_shape, const VmapDimVector& weight_grad_padded_shape, const VmapDimVector& weight_grad_shape) {
+  // helper function that produces a batched gradient for prelu using a decomposition inspired by the AOTAutograd ones
+  const auto input_grad_collector = at::where(self > 0, grad_out, weight * grad_out);
+  const auto input_grad = native::sum_to_size(input_grad_collector, self_grad_shape);
+  const auto weight_grad_collector = at::where(self > 0, at::zeros(1, self.options()), self * grad_out);
+  const auto weight_grad_collector_2 = native::sum_to_size(weight_grad_collector, weight_grad_padded_shape);
+  const auto weight_grad = weight_grad_collector_2.view(weight_grad_shape);
+  return std::make_tuple(input_grad, weight_grad);
+}
+
+std::tuple<Tensor,optional<int64_t>,Tensor,optional<int64_t>> prelu_backward_batch_rule(
+    const Tensor& grad_out, optional<int64_t> grad_out_bdim,
+    const Tensor& self, optional<int64_t> self_bdim,
+    const Tensor& weight, optional<int64_t> weight_bdim) {
+  const auto batch_size = get_bdim_size3(grad_out, grad_out_bdim, self, self_bdim, weight, weight_bdim);
+  const auto grad_out_ = moveBatchDimToFront(grad_out, grad_out_bdim);
+  const auto self_ = moveBatchDimToFront(self, self_bdim);
+  const auto self_size_with_bdim = ensure_shape_with_bdim(self_, self_bdim.has_value(), batch_size);
+  if (!weight_bdim && weight.dim() == 0) {
+    VmapDimVector weight_grad_shape(1, batch_size);
+    VmapDimVector weight_grad_shape_padded(self_bdim.has_value() ? self.dim() : self.dim() + 1, 1);
+    weight_grad_shape_padded[0] = batch_size;
+    const auto grads = prelu_backward_batched(grad_out_, self_, weight, self_size_with_bdim, weight_grad_shape_padded, weight_grad_shape);
+    return std::make_tuple(std::get<0>(grads), 0, std::get<1>(grads), 0);
+  }
+  const auto weight_ = moveBatchDimToFront(weight, weight_bdim);
+  auto weight_flatten = weight_;
+  if (weight_flatten.dim() > 1) {
+    // for an input [N, C, ...]
+    // weight can be a non-vector but the total number of elements must be the same as C
+    weight_flatten = at::flatten(weight_flatten, weight_bdim.has_value() ? 1 : 0, -1);
+  }
+
+  const int64_t self_logical_rank = rankWithoutBatchDim(self, self_bdim);
+  VmapDimVector new_shape(weight_flatten.sizes().begin(), weight_flatten.sizes().end());
+  const int64_t final_size = weight_bdim ? (self_logical_rank + 1) : self_logical_rank;
+  new_shape.reserve(final_size);
+
+  if (weight_flatten.dim() == 2 || !weight_bdim) {
+    // if weight (without batching) is not a scalar, its size must match the "channel dimension" of input. To do the
+    // decomposition, we pad the weight to
+
+    // copies checks from prelu if the weight (without vmap) is not a scalar
+    TORCH_CHECK(self_logical_rank > 0, "Not allow zero-dim input tensor.");
+
+    int64_t channel_size = 1; // channel_size default to 1
+    if (self_logical_rank > 1) {
+      channel_size = self_.size(self_bdim.has_value() ? 2 : 1);
+    }
+
+    const auto weight_num = weight_flatten.size(-1);
+    TORCH_CHECK(channel_size == weight_num,
+      "Mismatch of parameter numbers and input channel size. Found parameter numbers = ", weight_num,
+      " and channel size = ", channel_size, ".");
+
+    // pads to the left so that the flattened shape matches up with the channel
+    if (!weight_bdim) {
+      new_shape.insert(new_shape.begin(), 1); 
+    } else {
+      new_shape.insert(new_shape.begin() + 1, 1);
+    }
+  }
+
+  for (int64_t i = new_shape.size(); i < final_size; i ++) {
+    new_shape.push_back(1);
+  }
+  // weight grad does not depend on weight values. It is batched iff grad_out or self are batched
+  const auto weight_grad_is_batched = grad_out_bdim.has_value() || self_bdim.has_value();
+
+  const auto weight_padded = weight_flatten.view(new_shape);
+  const auto weight_grad_shape = shape_maybe_with_bdim(weight_, weight_grad_is_batched, weight_bdim.has_value(), batch_size);
+  const auto weight_padded_grad_shape = shape_maybe_with_bdim(weight_padded, weight_grad_is_batched, weight_bdim.has_value(), batch_size);
+
+  const auto grads = prelu_backward_batched(grad_out_, self_, weight_padded, self_size_with_bdim, weight_padded_grad_shape, weight_grad_shape);
+  return std::make_tuple(std::get<0>(grads), 0, std::get<1>(grads), (weight_grad_is_batched ? optional<int64_t>(0) : nullopt));
+}
+
+TORCH_LIBRARY_IMPL(aten, FT_BATCHED_KEY, m) {
+  VMAP_SUPPORT(prelu, prelu_batch_rule)
+  VMAP_SUPPORT(prelu_backward, prelu_backward_batch_rule)
+}
+}} // namespace at::functorch


Test patch:
diff --git a/test/test_ops.py b/test/test_ops.py
index 79135c69e..b0b4968e2 100644
--- a/test/test_ops.py
+++ b/test/test_ops.py
@@ -859,7 +859,6 @@ def test_vmapjvpall(self, device, dtype, op):
         xfail('nn.functional.huber_loss'),
         xfail('nn.functional.poisson_nll_loss'),
         xfail('nn.functional.bilinear'),
-        xfail('nn.functional.prelu'),
         xfail('nn.functional.glu'),
         xfail('nn.functional.fractional_max_pool3d'),
         xfail('as_strided'),
diff --git a/test/test_vmap.py b/test/test_vmap.py
index 26c186f82..83d623e05 100644
--- a/test/test_vmap.py
+++ b/test/test_vmap.py
@@ -3230,7 +3230,6 @@ def test_vmap_exhaustive(self, device, dtype, op):
         xfail('stft'),
         xfail('linalg.solve_triangular'),
         xfail('nn.functional.glu'),
-        xfail('nn.functional.prelu'),
         xfail('isclose'),
         xfail('nn.functional.fractional_max_pool3d'),
         xfail('nn.functional.bilinear'),