Fix CHROMIUM_EVENT_LOG being none

Summary:
X-link: pytorch/pytorch#154258

It turns out if you import something that's None at import time in python, and later update the value, the one you imported stays none:

```
import torch
from torch._dynamo.utils import CHROMIUM_EVENT_LOG
class Foo:
  pass
torch._dynamo.utils.CHROMIUM_EVENT_LOG =  Foo()

print(CHROMIUM_EVENT_LOG) # None
```

This fixes teh bug so we get AOTAutogradCache instant events again
ghstack-source-id: 285913063
exported-using-ghexport

Reviewed By: Myrthan, oulgen

Differential Revision: D75305770

fbshipit-source-id: b2e1692b80216ae59e7409472565a903ffbc64d2

Author: jamesjwu

userbenchmark/dynamo/dynamobench/_dynamo/utils.py

@@ -592,7 +592,7 @@ def try_add_pt2_compile(event_name: str, **metadata: object):
         or ChromiumEventLogger is not initialized.
         This function is syntactic sugar for chromium_event_logger().try_add_event_data.
         """
-        if CHROMIUM_EVENT_LOG is None:
+        if not chromium_event_log_active():
             return
         chromium_log = get_chromium_event_logger()
         chromium_log.try_add_event_data(event_name, **metadata)
@@ -602,7 +602,7 @@ def try_(method_fn, *args, **kwargs):
         """
         Special function that quietly runs a given method, returning if CHROMIUM_EVENT_LOG is None or metrics context is not set
         """
-        if CHROMIUM_EVENT_LOG is None:
+        if not chromium_event_log_active():
             return
         metrics_context = get_metrics_context()
         if not metrics_context.in_progress():
@@ -1923,6 +1923,11 @@ def get_chromium_event_logger() -> ChromiumEventLogger:
     return CHROMIUM_EVENT_LOG
 
 
+def chromium_event_log_active() -> bool:
+    global CHROMIUM_EVENT_LOG
+    return CHROMIUM_EVENT_LOG is not None
+
+
 @contextmanager
 def chromium_event_timed(
     event_name: str,