trunk/1190095994b3b3913e3d2e60d63219cf786ba3b4: [FakeTensor] Add hinted symbolic storage size metadata (#183839)

Trace tooling serializes FakeTensor storage metadata to JSON. When the

storage size is a hinted SymInt, the trace can have both useful pieces of

information:

• the symbolic storage expression, which preserves provenance for downstream
  
  trace consumers;


• the optimization hint, which gives diagnostic/policy tooling a concrete
  
  expected extent when one was explicitly provided.

Keep the existing size field symbolic for symbolic storage sizes and add a

separate size_hint field only when every free symbol in the storage-size

expression has an explicit optimization hint override. This preserves the

existing symbolic trace contract while exposing concrete policy metadata

without specializing tensor shapes or changing runtime semantics.

Fixes #183835

Test Plan:

python test/test_fake_tensor.py FakeTensorTest.test_meta_storage_trace_uses_hint_for_symbolic_size -q

lintrunner --config=.lintrunner.toml torch/_subclasses/meta_utils.py test/test_fake_tensor.py

Pull Request resolved: #183839

Approved by: https://github.com/ezyang, https://github.com/laithsakka