Change Dynamo's custom ops warning message to be less spammy (#128456)

Summary:
This is a short-term fix (for 2.4). In the longer term we should
fix pytorch/pytorch#128430

The problem is that warnings.warn that are inside Dynamo print
all the time. Python warnings are supposed to print once, unless their
cache is reset: Dynamo ends up resetting that cache everytime it runs.

As a workaround we provide our own warn_once cache that is keyed on the
warning msg. I am not worried about this increasing memory usage because
that's effectively what python's warnings.warn cache does.

X-link: pytorch/pytorch#128456
Approved by: https://github.com/anijain2305

Reviewed By: clee2000

Differential Revision: D58501328

Pulled By: zou3519

fbshipit-source-id: 99dcfddfae27de2f1de6e9685aa990a738531199

userbenchmark/dynamo/dynamobench/_dynamo/utils.py

@@ -23,6 +23,7 @@
 import time
 import types
 import typing
+import warnings
 import weakref
 from contextlib import contextmanager
 from functools import lru_cache, wraps
@@ -2751,3 +2752,18 @@ def __init__(self, s):
 
     def __repr__(self):
         return self.s
+
+
+warn_once_cache: Set[str] = set()
+
+
+def warn_once(msg, stacklevel=1):
+    # Dynamo causes all warnings.warn (in user code and in Dynamo code) to print all the time.
+    # https://github.com/pytorch/pytorch/issues/128427.
+    # warn_once is a workaround: if the msg has been warned on before, then we will not
+    # warn again.
+    # NB: it's totally ok to store a cache of all the strings: this is what warnings.warn does as well.
+    if msg in warn_once_cache:
+        return
+    warn_once_cache.add(msg)
+    warnings.warn(msg, stacklevel=stacklevel + 1)