[dynamo][ez] Initialize tracer_output to None by default. (pytorch#167366)

[dynamo][ez] Initialize tracer_output to None by default. (pytorch#163169)

Summary:
In edge cases, tracer_output can be left unset if there's double exception raised which causes the following issue:
```
UnboundLocalError: local variable 'tracer_output' referenced before assignment
```

Default initialize this variable so that it's always present.

Test Plan:
CI

Rollback Plan:

Differential Revision: D82652815

Pull Request resolved: pytorch#163169
Approved by: https://github.com/tugsbayasgalan

(cherry picked from commit 6189a5f)

Co-authored-by: Zhengxu Chen <[email protected]>

Author: pytorchbot

torch/_dynamo/convert_frame.py

@@ -1429,6 +1429,7 @@ def format_func_info(code: CodeType) -> str:
         fail_user_frame_lineno: Optional[int] = None
         torch._dynamo.utils.ReinplaceCounters.clear()
         guarded_code = None
+        tracer_output = None
         try:
             guarded_code, tracer_output = compile_inner(code, one_graph, hooks)