CoreML model works with torch.jit.trace, but not torch.export.export

🐛 Describe the bug

In the attached python code, there is a model that works in CoreML with torch.jit.trace, but not with torch.export.export.

When running ct.convert from torch.export.export, an error is encountered ("ERROR - converting 'narrow' op (located at: 'narrow'): ... TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'"

Repro is located here: coreml_issue.txt

Versions

Collecting environment information...
PyTorch version: 2.7.0.dev20250311
Is debug build: False
CUDA used to build PyTorch: None
ROCM used to build PyTorch: N/A

OS: macOS 15.3.2 (arm64)
GCC version: Could not collect
Clang version: 16.0.0 (clang-1600.0.26.6)
CMake version: version 3.31.4
Libc version: N/A

Python version: 3.10.16 (main, Dec 11 2024, 10:22:29) [Clang 14.0.6 ] (64-bit runtime)
Python platform: macOS-15.3.2-arm64-arm-64bit
Is CUDA available: False
CUDA runtime version: No CUDA
CUDA_MODULE_LOADING set to: N/A
GPU models and configuration: No CUDA
Nvidia driver version: No CUDA
cuDNN version: No CUDA
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Apple M1 Pro

Versions of relevant libraries:
[pip3] executorch==0.6.0a0+247fb8d
[pip3] executorchcoreml==0.0.1
[pip3] flake8==6.1.0
[pip3] flake8-breakpoint==1.1.0
[pip3] flake8-bugbear==24.4.26
[pip3] flake8-comprehensions==3.14.0
[pip3] flake8-plugin-utils==1.3.3
[pip3] flake8-pyi==23.5.0
[pip3] mypy==1.14.1
[pip3] mypy-extensions==1.0.0
[pip3] numpy==2.0.0
[pip3] torch==2.7.0.dev20250311
[pip3] torchao==0.10.0+git
[pip3] torchaudio==2.6.0.dev20250311
[pip3] torchsr==1.0.4
[pip3] torchtune==0.6.0.dev20250131+cpu
[pip3] torchvision==0.22.0.dev20250311
[conda] executorch 0.6.0a0+247fb8d pypi_0 pypi
[conda] executorchcoreml 0.0.1 pypi_0 pypi
[conda] numpy 2.0.0 pypi_0 pypi
[conda] torch 2.7.0.dev20250311 pypi_0 pypi
[conda] torchao 0.9.0+git04f3f03d pypi_0 pypi
[conda] torchaudio 2.6.0.dev20250311 pypi_0 pypi
[conda] torchfix 0.6.0 pypi_0 pypi
[conda] torchsr 1.0.4 pypi_0 pypi
[conda] torchtune 0.6.0.dev20250131+cpu pypi_0 pypi
[conda] torchvision 0.22.0.dev20250311 pypi_0 pypi

cc @kimishpatel @YifanShenSZ @cymbalrush

Start with the attached coreml_issue.txt reproduction and compare the torch.jit.trace and torch.export.export paths entering ct.convert. Investigate the reported narrow conversion error, especially the NoneType-plus-int failure. Done means the exported model converts successfully while retaining the behavior already shown to work with torch.jit.trace.