Add generic fx-graph serializer (NFXG)

[metascroy]

Summary:
Serialize a torch.fx graph to a standalone flatbuffer (native_graph.fbs, id
"NFXG") and back: a topological node list with a tagged Argument union, tensor
metadata side table, and by-reference constants (ConstantRef.fqn) whose bytes are
shipped separately. Covers input/output classification, symbolic dims in list
args, and a validate_graph() completeness check. Constants are never inlined.

Reviewed By: SS-JIA

Differential Revision: D111753288

[pytorch-bot]

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❌ 2 New Failures

As of commit 065febf with merge base 8077bb6 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2f7a8a9d81d02a53789e162c670c1b0036a7f39bf2f6876bcfa30c2bf77af2cc /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7a5ad7a03c3a7def4f932a169e15efa10c0fdb2af7f03e1c3a3bb722972bcbcb /exec failed with exit code 1

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[meta-codesync]

@metascroy has exported this pull request. If you are a Meta employee, you can view the originating Diff in D111753288.

[SS-JIA]

Review automatically exported from Phabricator review in Meta.

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