[BE] Save attributes for CppCompileError for pickleing (pytorch#160294)

Differential Revision: [D79977408](https://our.internmc.facebook.com/intern/diff/D79977408/)

Context:
When testing cutlass backend and used autotune with subproc, sometimes I would see C++ compilation error (expected) followed by
```
Traceback (most recent call last):
  File "/torch/_inductor/autotune_process.py", line 175, in get
    result = TuningProcess.recv(self.read_pipe)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/torch/_inductor/autotune_process.py", line 99, in recv
    return pickle.load(read_pipe)
           ^^^^^^^^^^^^^^^^^^^^^^
TypeError: CppCompileError.__init__() missing 1 required positional argument: 'output'
```
which is unexpected. After asking claude, it seems

> Now I can see the issue. The `CppCompileError` class requires two arguments: `cmd` (a list of strings) and `output` (a string). However, when exceptions are being pickled and unpickled across process boundaries, the pickling process might not be preserving the constructor arguments correctly.
>
> The problem is likely that when a `CppCompileError` is raised in the subprocess and then pickled/unpickled through the `recv` function, the unpickling process is trying to reconstruct the exception but doesn't have the required constructor arguments.
>
> The issue is clear now. The `CppCompileError` class doesn't have custom pickle methods (`__reduce__`, `__getstate__`, `__setstate__`), so when it's pickled and unpickled across process boundaries, Python's default pickling mechanism tries to reconstruct it but fails because it doesn't preserve the constructor arguments properly.
>
> The solution is to add a `__reduce__` method to the `CppCompileError` class to ensure it can be properly pickled and unpickled. Let me implement this fix:

Adding these seem to help.

fbcode repro: [D79977541](https://www.internalfb.com/diff/D79977541)

Pull Request resolved: pytorch#160294
Approved by: https://github.com/masnesral

Author: henrylhtsang

torch/_inductor/exc.py

@@ -92,6 +92,9 @@ def __init__(self, cmd: list[str], output: str) -> None:
         if isinstance(output, bytes):
             output = output.decode("utf-8")
 
+        self.cmd = cmd
+        self.output = output
+
         super().__init__(
             textwrap.dedent(
                 """
@@ -108,6 +111,9 @@ def __init__(self, cmd: list[str], output: str) -> None:
             .format(cmd=" ".join(cmd), output=output)
         )
 
+    def __reduce__(self) -> tuple[type, tuple[list[str], str]]:
+        return (self.__class__, (self.cmd, self.output))
+
 
 class CUDACompileError(CppCompileError):
     pass