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[Nested Tensor]Clean up offsets #84145

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[Nested Tensor]Clean up offsets #84145

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29 changes: 19 additions & 10 deletions aten/src/ATen/NestedTensorImpl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,9 @@
#include <c10/util/Exception.h>
#include <c10/core/TensorImpl.h>

#include <numeric>
#include <functional>

namespace {
inline void validate_nested_tensor_metadata(
const at::Tensor& nested_sizes,
Expand All @@ -20,7 +23,7 @@ inline void validate_nested_tensor_metadata(
TORCH_INTERNAL_ASSERT(nested_sizes.sizes() == nested_strides.sizes());
TORCH_INTERNAL_ASSERT(
(size_dim == 0 && (int64_t)offsets.empty()) ||
(size_dim == 2 && nested_sizes.size(0) == (int64_t)offsets.size()));
(size_dim == 2 && nested_sizes.size(0) + 1 == (int64_t)offsets.size()));
}

} // namespace
Expand Down Expand Up @@ -85,27 +88,33 @@ inline at::Tensor construct_nested_stride_tensor(const at::Tensor& sizes) {
return strides;
}

// assume contiguous, we can construct offsets from size
/**
* Create a vector of offsets assuming the nested tensor is contiguous
*
* This function iterates over the implicit ntensor outer dimension
* populating a vector with the num_elements in each implicit tensor.
* The first element is always 0 and the length of the returned vector
* is n_tensor + 1.
* num_elements in ntensor[i] = offsets[i+1] - offsets[i]
*
* @return A vector of offsets
*/
inline std::vector<int64_t> construct_offsets(const at::Tensor& sizes) {
// empty `sizes` means empty nested tensor, so return empty strides
if (sizes.dim() == 0) {
return std::vector<int64_t>();
}
int64_t ntensors = sizes.size(0),
orig_dim = sizes.size(1);
std::vector<int64_t> offsets(ntensors);
int64_t ntensors = sizes.size(0), orig_dim = sizes.size(1);
std::vector<int64_t> offsets(ntensors + 1);
// nesting scalars has easy offsets
if (orig_dim == 0) {
std::iota(offsets.begin(), offsets.end(), 0);
return offsets;
}
const int64_t* sizes_ptr = sizes.data_ptr<int64_t>();
offsets[0] = 0;
for (int64_t i = 0; i < ntensors - 1; i++) {
int64_t row_product = sizes_ptr[0];
for (int64_t j = 1; j < orig_dim; j++) {
row_product *= sizes_ptr[j];
}
for (const auto i : c10::irange(ntensors)) {
const int64_t row_product = std::accumulate(sizes_ptr, sizes_ptr + orig_dim, 1, std::multiplies<int64_t>());
offsets[i + 1] = offsets[i] + row_product;
sizes_ptr += orig_dim;
}
Expand Down