…guards (pytorch#186993)

Summary:

`ExportedProgram.module()` builds a `_guards_fn` submodule that re-asserts the exported shape guards. For each assert's human-readable error message, `_convert_guards_code_to_fn` (in `torch/export/_unlift.py`) pretty-prints the guard via `ast.unparse(ast.parse(shadow))`. Both `ast.parse` and `ast.unparse` recurse once per AST node, so a guard whose expression is very deeply nested -- e.g. a sum over many symbolic sizes, as produced when exporting a recommendation model with `auto_dynamic_shapes` over hundreds of jagged/KJT features -- exceeds Python's recursion limit and raises `RecursionError`, aborting the entire export (including standalone publish, which reaches this code via `run_decompositions()` -> `module()`).

Root cause: the `ast.unparse(ast.parse(...))` round-trip is purely cosmetic; as the existing comment states, it "is not necessary for correctness, just deemed desirable" -- it only normalizes redundant parentheses in the assert error string. The executed runtime check uses the separate `actual` expression and does not depend on the pretty-printed `shadow`, so a deep guard should never be fatal.

Fix: wrap the normalization in `try/except RecursionError` and fall back to the un-normalized guard string. The emitted runtime assert is unchanged; only the readability of the guard-failure message degrades slightly in the rare deep-guard case.

Test Plan:
Built custom aps package and publish f1096406197

Added `test_guards_fn_recovers_from_unparse_recursion_error`, which mocks `ast.unparse` to raise `RecursionError` and asserts `_convert_guards_code_to_fn` still returns a guards fn instead of propagating the error. A mock is used rather than a genuinely deep expression because the test target is ASAN-instrumented, where deep `ast.parse`/`compile` recursion can abort the process before the pure-Python `RecursionError` is reached.

```
buck2 test fbcode//caffe2/test:test_export -- --regex 'test_guards_fn_recovers_from_unparse_recursion_error'
```

After the fix: `Pass 11. Fail 0. Fatal 0.` (the test is fanned out across export modes: strict, nonstrict, serdes, retraceability, cpp_serdes, training_ir, nativert, ...). Before the fix the same test fails with `RecursionError: maximum recursion depth exceeded` at `_unlift.py` (`Pass 0, Fail 11`).

Authored with the assistance of an AI coding assistant.

Reviewed By: jijunyan, sophielin508

Differential Revision: D108111211