Add Python APIs for PTD tensor dictionaries

[Kilbex]

Summary

Adds public save_ptd and load_ptd Python helpers for saving and loading PTD tensor dictionaries without constructing DataPayload objects directly. The helpers validate supported CPU tensors, preserve contiguous and channels-last layouts, normalize other strided layouts while preserving shape and values, and reject malformed tensor entries.

Includes the review fixes from #21236: saved entries use the default FlatTensorConfig segment alignment for C++ mmap compatibility, optional torch dtypes are skipped when unavailable, and the API documentation explains layout normalization.

Replaces #21236 following the request to open a new PR. The original fork was deleted and recreated, so the original PR could not be reopened. This PR uses the updated Kilbex:python-ptd-tensor-dicts branch, including commit 788378f, so CI can run on the current changes.

Test plan

PYTHONPATH=.. python -m pytest extension/flat_tensor/test/test_serialize.py -q — 13 tests and 20 subtests passed.

lintrunner extension/flat_tensor/README.md extension/flat_tensor/serialize/BUCK extension/flat_tensor/serialize/__init__.py extension/flat_tensor/serialize/serialize.py extension/flat_tensor/test/test_serialize.py — no lint issues.

The earlier C++ smoke-test results cover FlatTensorDataMap with MmapDataLoader, 128-byte alignment, direct mapped-buffer aliases, mixed dtypes, and an empty tensor.

Implementation authored with Claude. Replacement PR prepared with Codex.

[pytorch-bot]

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[github-actions]

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[nil-is-all]

Hi @Kilbex, could you resolve branch conflicts and ping once ready for review? Thanks for putting up a new PR!

[Kilbex]

@nil-is-all The branch is updated with the latest main, and GitHub confirms there are no merge conflicts. The update is pushed and ready for review.

Validation: PYTHONPATH=.. python -m pytest extension/flat_tensor/test/test_serialize.py -q — 16 tests and 20 subtests passed. Lint passed for all five changed files. Remote checks have started on the updated commit.

Thanks!