Generate source tarball with PEP 517 conform build tools instead of the custom routine in place right now.
Closes#150461.
The current procedure for generating the source tarball consists in creation of a source tree by manual copying and pruning of source files.
This PR replaces that with a call to the standard [build tool](https://build.pypa.io/en/stable/), which works with the build backend to produce an sdist. For that to work correctly, the build backend also needs to be configured. In the case of Pytorch, the backend currently is (the legacy version of) the setuptools backend, the source dist part of which is mostly configured via the `MANIFEST.in` file.
The resulting source distribution can be used to install directly from source with `pip install ./torch-{version}.tar.gz` or to build wheels directly from source with `pip wheel ./torch-{version}.tar.gz`; both should be considered experimental for now.
## Issues
### sdist name
According to PEP 517, the name of the source distribution file must coincide with the project name, or [more precisely](https://peps.python.org/pep-0517/#source-distributions), the source distribution of a project that generates `{NAME}-{...}.whl` wheels are required to be named `{NAME}-{...}.tar.gz`. Currently, the source tarball is called `pytorch-{...}.tar.gz`, but the generated wheels and python package are called `torch-{...}`.
### Symbolic Links
The source tree at the moment contains a small number of symbolic links. This [has been seen as problematic](https://github.com/pypa/pip/issues/5919) largely because of lack of support on Windows, but also because of [a problem in setuptools](https://github.com/pypa/setuptools/issues/4937). Particularly unfortunate is a circular symlink in the third party `ittapi` module, which can not be resolved by replacing it with a copy.
PEP 721 (now integrated in the [Source Distribution Format Specification](https://packaging.python.org/en/latest/specifications/source-distribution-format/#source-distribution-archive-features)) allows for symbolic links, but only if they don't point outside the destination directory and if they don't contain `../` in their target.
The list of symbolic links currently is as follows:
<details>
|source|target|problem|solution|
|-|-|-|-|
| `.dockerignore` | `.gitignore` | ✅ ok (individual file) ||
| `docs/requirements.txt` | `../.ci/docker/requirements-docs.txt` |❗`..` in target|swap source and target[^1]|
| `functorch/docs/source/notebooks` | `../../notebooks/` |❗`..` in target|swap source and target[^1]|
| `.github/ci_commit_pins/triton.txt` | `../../.ci/docker/ci_commit_pins/triton.txt` | ✅ ok (omitted from sdist)||
| `third_party/flatbuffers/docs/source/CONTRIBUTING.md` | `../../CONTRIBUTING.md` |❗`..` in target|omit from sdist[^2]|
| `third_party/flatbuffers/java/src/test/java/DictionaryLookup` | `../../../../tests/DictionaryLookup` |❗`..` in target|omit from sdist[^3]|
| `third_party/flatbuffers/java/src/test/java/MyGame` | `../../../../tests/MyGame` |❗`..` in target|omit from sdist[^3]|
| `third_party/flatbuffers/java/src/test/java/NamespaceA` | `../../../../tests/namespace_test/NamespaceA` |❗`..` in target|omit from sdist[^3]|
| `third_party/flatbuffers/java/src/test/java/NamespaceC` | `../../../../tests/namespace_test/NamespaceC` |❗`..` in target|omit from sdist[^3]|
| `third_party/flatbuffers/java/src/test/java/optional_scalars` | `../../../../tests/optional_scalars` |❗`..` in target|omit from sdist[^3]|
| `third_party/flatbuffers/java/src/test/java/union_vector` | `../../../../tests/union_vector` |❗`..` in target|omit from sdist[^3]|
| `third_party/flatbuffers/kotlin/benchmark/src/jvmMain/java` | `../../../../java/src/main/java` |❗`..` in target|omit from sdist[^3]|
| `third_party/ittapi/rust/ittapi-sys/c-library` | `../../` |❗`..` in target|omit from sdist[^4]|
| `third_party/ittapi/rust/ittapi-sys/LICENSES` | `../../LICENSES` |❗`..` in target|omit from sdist[^4]|
| `third_party/opentelemetry-cpp/buildscripts/pre-merge-commit` | `./pre-commit` |✅ ok (individual file)||
| `third_party/opentelemetry-cpp/third_party/prometheus-cpp/cmake/project-import-cmake/sample_client.cc` | `../../push/tests/integration/sample_client.cc` |❗`..` in target|omit from sdist[^5]|
| `third_party/opentelemetry-cpp/third_party/prometheus-cpp/cmake/project-import-cmake/sample_server.cc` | `../../pull/tests/integration/sample_server.cc` |❗`..` in target|omit from sdist[^5]|
| `third_party/opentelemetry-cpp/third_party/prometheus-cpp/cmake/project-import-pkgconfig/sample_client.cc` | `../../push/tests/integration/sample_client.cc` |❗`..` in target|omit from sdist[^5]|
| `third_party/opentelemetry-cpp/third_party/prometheus-cpp/cmake/project-import-pkgconfig/sample_server.cc` | `../../pull/tests/integration/sample_server.cc` |❗`..` in target|omit from sdist[^5]|
| `third_party/XNNPACK/tools/xngen` | `xngen.py` | ✅ ok (individual file)||
</details>
The introduction of symbolic links inside the `.ci/docker` folder creates a new problem, however, because Docker's `COPY` command does not allow symlinks in this way. We work around that by using `tar ch` to dereference the symlinks before handing them over to `docker build`.
[^1]: These resources can be naturally considered to be part of the docs, so moving the actual files into the place of the current symlinks and replacing them with (unproblematic) symlinks can be said to improve semantics as well.
[^2]: The flatbuffers docs already actually use the original file, not the symlink and in the most recent releases, starting from flatbuffers-25.1.21 the symlink is replaced by the actual file thanks to a documentation overhaul.
[^3]: These resources are flatbuffers tests for java and kotlin and can be omitted from our sdist.
[^4]: We don't need to ship the rust bindings for ittapi.
[^5]: These are demonstration examples for how to link to prometheus-cpp using cmake and can be omitted.
### Nccl
Nccl used to be included as a submodule. However, with #146073 (first released in v2.7.0-rc1), the submodule was removed and replaced with a build time checkout procedure in `tools/build_pytorch_libs.py`, which checks out the required version of nccl from the upstream repository based on a commit pin recorded in `.ci/docker/ci_commit_pins/nccl-cu{11,12}.txt`.
This means that a crucial third party dependency is missing from the source distribution and as the `.ci` folder is omitted from the source distribution, it is not possible to use the build time download.
However, it *is* possible to use a system provided Nccl using the `USE_SYSTEM_NCCL` environment variable, which now also is the default for the official Pytorch wheels.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/152098
Approved by: https://github.com/atalman
