Fix constant_prop_pass fqn collision with state_dict (#23266)

[pcwu0329]

Summary:

replace_with_constant_node names propagated constants
_prop_tensor_constant{len(exported_program.constants)}, then checks for a
collision. Both the check and the max-suffix rescan look only at
exported_program.constants.

A program re-entering this pass can already carry a _prop_tensor_constant* in
exported_program.state_dict from an earlier run. That name is invisible to
both, so the pass reuses it and writes its tensor into constants alongside the
stale state_dict entry. Emission then resolves the placeholder against
state_dict before constants (exir/emit/_emitter.py:2044), picks up the
stale tensor, and fails the size check at _emitter.py:513:

InternalError: Tensor spec has buffer of size 48, but expected nbytes of 4

The spec is correct at 4 bytes; the tensor bound to it is the wrong one. Note
the existing comment already reads "If prop_constant_tensor_fqn already exists
in the state dict" -- the implementation never matched that intent.

Widen the collision check and the suffix rescan to span state_dict as well as
constants.

Observed on a Turing/Coleman hand-tracking model compiled with
fx_options.allow_quant_dequant_on_tce=True, which folds a scalar 1 from a
1 - valid_multi_view expression whose name was already taken in state_dict.
Instrumenting the pass showed the collision directly:

fqn=_prop_tensor_constant20 new_nbytes=4
already_in_constants=False already_in_state_dict=True

With the fix the model compiles to a .pte (3 TCE delegates, 84.19% delegation);
without it emission aborts.

Differential Revision: D122298673

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 0fe4ca4 with merge base 273cb33 ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh) (trunk failure)
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• ✅ login: pcwu0329 / name: Po-Chen Wu (a0df710)

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