Repository: pytorch
Issue: fx::GraphModule::to_folder doesn't seem to handle _PyTreeCodeGen correctly.
Labels: module: fx,oncall: export

### 🐛 Describe the bug

I am trying to serialize a GraphModule using `to_folder` (I am assuming this is the correct way to serialize a torch.export'ed GraphModule - if not, please correct me). When I try to reload the module, I see that it is missing the settings for `self._in_spec` and `self._out_spec` which causes it to fail. Simple example below.

```python
import torch
import sys

FLOAT_LINEAR_RELU_EXAMPLE = (
        torch.randn(4, 4), torch.randn(4, 4), torch.randn(4)
        )

class LinearPattern(torch.nn.Module):

    def __init__(self, relu_en):
        super().__init__()
        self.relu_en = relu_en

    def forward(self, x, w, b):
        
        y = torch.ops.aten.linear(x, w, b)
        if self.relu_en:
            y = torch.ops.linear.relu(y)
        return y

if __name__ == "__main__":

    # export the module.
    prog = torch.export.export(
            LinearPattern(False),
            FLOAT_LINEAR_RELU_EXAMPLE
            )
    # serialize to a folder.
    prog.module().to_folder('prog_save')

    # import and run.
    sys.path.append('.')
    import prog_save
    g = prog_save.FxModule()
    g(*FLOAT_LINEAR_RELU_EXAMPLE)

```
I see this error when I run the above code:

```
Traceback (most recent call last):
  File "tests/repro/to_folder.py", line 35, in <module>
    g(*FLOAT_LINEAR_RELU_EXAMPLE)
  File "/local/mnt/workspace/.executorch/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1519, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/local/mnt/workspace/.executorch/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1528, in _call_impl
    return forward_call(*args, **kwargs)
  File "<snip>/prog_save/module.py", line 20, in forward
    arg0_1, arg1_1, arg2_1, = fx_pytree.tree_flatten_spec(([arg_0, arg_1, arg_2], {}), self._in_spec)
  File "/local/mnt/workspace/.executorch/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1696, in __getattr__
    raise AttributeError(f"'{type(self).__name__}' object has no attribute '{name}'")
AttributeError: 'FxModule' object has no attribute '_in_spec'
```

When I inspect the generated code in `prog_save/module.py`, I see the following:

```python
class FxModule(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.load_state_dict(torch.load(r'prog_save/state_dict.pt'))

    
    
    def forward(self, arg_0, arg_1, arg_2):
        arg0_1, arg1_1, arg2_1, = fx_pytree.tree_flatten_spec(([arg_0, arg_1, arg_2], {}), self._in_spec)
        permute = torch.ops.aten.permute.default(arg1_1, [1, 0]);  arg1_1 = None
        addmm = torch.ops.aten.addmm.default(arg2_1, arg0_1, permute);  arg2_1 = arg0_1 = permute = None
        return pytree.tree_unflatten((addmm,), self._out_spec)
```

The `forward` function seems to refer to `self._in_spec` while its never initialized anywhere.

It seems like `to_folder` is missing the following logic - i.e., this code needs to be appended to `to_folder`.
https://github.com/pytorch/pytorch/blob/11e4a9266d15c7c663ce9cb4dff4f0e962f8072f/torch/fx/graph_module.py#L714-L716

Please let me know if I missed anything here, or if I can provide more information. If there is any other alternate way to serialize/deserialize a `GraphModule` or a torch exported program into a file, please let me know.

Thanks in Advance.

### Versions

PyTorch version: 2.2.0.dev20231010+cpu
Is debug build: False
CUDA used to build PyTorch: Could not collect
ROCM used to build PyTorch: N/A

OS: Ubuntu 20.04.6 LTS (x86_64)
GCC version: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Clang version: Could not collect
CMake version: version 3.28.1
Libc version: glibc-2.31

Python version: 3.8.10 (default, May 26 2023, 14:05:08)  [GCC 9.4.0] (64-bit runtime)
Python platform: Linux-5.4.0-146-generic-x86_64-with-glibc2.29
Is CUDA available: False
CUDA runtime version: Could not collect
CUDA_MODULE_LOADING set to: N/A
GPU models and configuration: GPU 0: GRID A100D-10C
Nvidia driver version: 525.60.13
cuDNN version: Could not collect
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

Versions of relevant libraries:
[pip3] executorch==0.1.0
[pip3] numpy==1.24.4
[pip3] torch==2.2.0.dev20231010+cpu
[pip3] torchaudio==2.2.0.dev20231010+cpu
[pip3] torchsr==1.0.4
[pip3] torchvision==0.17.0.dev20231010+cpu
[conda] Could not collect


cc @ezyang @SherlockNoMad @EikanWang @jgong5 @wenzhe-nrv @msaroufim @bdhirsh @anijain2305 @zou3519 @avikchaudhuri @gmagogsfm @zhxchen17 @tugsbayasgalan @angelayi @suo @ydwu4