[MegaCache] Return None on no compilation (#151921)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/151921
Approved by: https://github.com/jamesjwu

test/inductor/test_codecache.py

@@ -584,6 +584,9 @@ class TestFxGraphCache(TestCase):
             f(torch.randn(2, 6))
             self.assertEqual(backend.frame_count, 1)
 
+    def test_cache_hot_load_empty(self):
+        self.assertIsNone(torch.compiler.save_cache_artifacts())
+
     @requires_triton()
     @config.patch({"fx_graph_cache": True})
     @config.patch({"fx_graph_remote_cache": False})

torch/compiler/_cache.py

@@ -85,6 +85,14 @@ class CacheInfo:
         self.aot_autograd_artifacts.clear()
         self.pgo_artifacts.clear()
 
+    def empty(self) -> bool:
+        return not (
+            self.inductor_artifacts
+            or self.autotune_artifacts
+            or self.aot_autograd_artifacts
+            or self.pgo_artifacts
+        )
+
 
 class CacheArtifactManager:
     """
@@ -181,6 +189,12 @@ class CacheArtifactManager:
         for artifact in cls._new_cache_artifacts:
             log.debug("saving: %s", artifact)
             cls._cache_info.add(artifact)
+
+        if cls._cache_info.empty():
+            # If there are not artifacts, dont just return bytes with
+            # version.
+            return None
+
         try:
             # We deep copy cls._cache_info since later compilations
             # can keep adding to cache_info