Add multi-platform torch example

This has been a useful starting point for me locally to repro pypi
downloader issues we see in our project.

Author: keith

examples/platform_specific_deps/.gitignore

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+bazel-*

examples/platform_specific_deps/BUILD.bazel

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+load("@pip//:requirements.bzl", "requirement")
+load("@rules_python//python:defs.bzl", "py_test")
+load("@rules_uv//uv:pip.bzl", "pip_compile")
+
+[
+    pip_compile(
+        name = "generate_{}_requirements".format(arch),
+        args = [
+            "--emit-index-url",
+            "--index-strategy=unsafe-best-match",  # NOTE: required because torch's index contains requests and that is preferred over pypi
+        ],
+        python_platform = arch,
+        requirements_in = "requirements.in",
+        requirements_txt = "{}-requirements.txt".format(arch),
+    )
+    for arch in [
+        "aarch64-unknown-linux-gnu",
+        "x86_64-unknown-linux-gnu",
+        "aarch64-apple-darwin",
+    ]
+]
+
+py_test(
+    name = "test",
+    srcs = ["test_load.py"],
+    main = "test_load.py",
+    deps = [
+        requirement("torch"),
+    ],
+)

examples/platform_specific_deps/MODULE.bazel

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+module(
+    name = "platform_specific_deps",
+    version = "0.0.0",
+    compatibility_level = 1,
+)
+
+bazel_dep(name = "bazel_skylib", version = "1.7.1")
+bazel_dep(name = "rules_python", version = "0.0.0")
+# TODO: Replace with builtin uv support if it supports platform specific requirements output
+bazel_dep(name = "rules_uv", version = "0.42.0")
+
+local_path_override(
+    module_name = "rules_python",
+    path = "../..",
+)
+
+python = use_extension("@rules_python//python/extensions:python.bzl", "python")
+python.toolchain(
+    python_version = "3.11",
+)
+
+pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")
+pip.parse(
+    hub_name = "pip",
+    python_version = "3.11",
+    requirements_by_platform = {
+        "//:x86_64-unknown-linux-gnu-requirements.txt": "linux_x86_64",
+        "//:aarch64-unknown-linux-gnu-requirements.txt": "linux_aarch64",
+    },
+    requirements_lock = "aarch64-apple-darwin-requirements.txt",
+)
+use_repo(pip, "pip")

examples/platform_specific_deps/aarch64-apple-darwin-requirements.txt

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+# This file was autogenerated by uv via the following command:
+#    bazel run @@//:generate_aarch64-apple-darwin_requirements
+--index-url https://pypi.org/simple
+--extra-index-url https://download.pytorch.org/whl/cpu
+
+filelock==3.16.1
+    # via torch
+fsspec==2024.10.0
+    # via torch
+jinja2==3.1.4
+    # via torch
+markupsafe==3.0.1
+    # via jinja2
+mpmath==1.3.0
+    # via sympy
+networkx==3.4.2
+    # via torch
+sympy==1.13.3
+    # via torch
+torch==2.4.1
+    # via -r requirements.in
+typing-extensions==4.12.2
+    # via torch

examples/platform_specific_deps/aarch64-unknown-linux-gnu-requirements.txt

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+# This file was autogenerated by uv via the following command:
+#    bazel run @@//:generate_aarch64-unknown-linux-gnu_requirements
+--index-url https://pypi.org/simple
+--extra-index-url https://download.pytorch.org/whl/cpu
+
+filelock==3.16.1
+    # via torch
+fsspec==2024.10.0
+    # via torch
+jinja2==3.1.4
+    # via torch
+markupsafe==3.0.1
+    # via jinja2
+mpmath==1.3.0
+    # via sympy
+networkx==3.4.2
+    # via torch
+sympy==1.13.3
+    # via torch
+torch==2.4.1
+    # via -r requirements.in
+typing-extensions==4.12.2
+    # via torch

examples/platform_specific_deps/requirements.in

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+--extra-index-url https://download.pytorch.org/whl/cpu
+torch==2.4.1; platform_machine != "x86_64"
+torch==2.4.1+cpu; platform_machine == "x86_64"

examples/platform_specific_deps/test_load.py

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+import torch  # This verifies the deps were loaded as expected for the current platform
+
+print("worked!")

examples/platform_specific_deps/x86_64-unknown-linux-gnu-requirements.txt

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+# This file was autogenerated by uv via the following command:
+#    bazel run @@//:generate_x86_64-unknown-linux-gnu_requirements
+--index-url https://pypi.org/simple
+--extra-index-url https://download.pytorch.org/whl/cpu
+
+filelock==3.16.1
+    # via torch
+fsspec==2024.10.0
+    # via torch
+jinja2==3.1.4
+    # via torch
+markupsafe==3.0.1
+    # via jinja2
+mpmath==1.3.0
+    # via sympy
+networkx==3.4.2
+    # via torch
+sympy==1.13.3
+    # via torch
+torch==2.4.1+cpu
+    # via -r requirements.in
+typing-extensions==4.12.2
+    # via torch