Lightweight at-most-once logging for API usage (#20745)

Summary:
Resubmit #20698 which got messed up.

Idea is that when PyTorch is used in a custom build environment (e.g. Facebook), it's useful to track usage of various APIs centrally. This PR introduces a simple very lightweight mechanism to do so - only first invocation of a trigger point would be logged. This is significantly more lightweight than #18235 and thus we can allow to put logging in e.g. TensorImpl.

Also adds an initial list of trigger points. Trigger points are added in such a way that no static initialization triggers them, i.e. just linking with libtorch.so will not cause any logging. Further suggestions of what to log are welcomed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20745

Differential Revision: D15429196

Pulled By: dzhulgakov

fbshipit-source-id: a5e41a709a65b7ebccc6b95f93854e583cf20aca

test/test_logging.py

@@ -0,0 +1,34 @@
+import os
+import sys
+import subprocess
+import torch
+from common_utils import TestCase, run_tests
+
+
+class LoggingTest(TestCase):
+    @staticmethod
+    def _runAndCaptureStderr(code):
+        env = os.environ.copy()
+        env["PYTORCH_API_USAGE_STDERR"] = "1"
+        pipes = subprocess.Popen(
+            [sys.executable, '-c', code],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            env=env)
+        return pipes.communicate()[1].decode('ascii')
+
+    def testApiUsage(self):
+        """
+        This test verifies that api usage logging is not triggered via static
+        initialization. Since it's triggered at first invocation only - we just
+        subprocess
+        """
+        s = self._runAndCaptureStderr("import torch")
+        self.assertRegexpMatches(s, "PYTORCH_API_USAGE.*import")
+        # import the shared library directly - it triggers static init but doesn't call anything
+        s = self._runAndCaptureStderr("from ctypes import CDLL; CDLL('{}')".format(torch._C.__file__))
+        self.assertNotRegexpMatches(s, "PYTORCH_API_USAGE")
+
+
+if __name__ == '__main__':
+    run_tests()