Create FileBaton to synchronize distributed JIT C++ extension builds (#6684)

* Create FileBaton to synchronize distributed JIT C++ extension builds

* Move FileBaton to its own file

* Autoformat code

* Respect verbose flag in cpp_extension._prepare_ldflags

torch/utils/file_baton.py

@@ -0,0 +1,47 @@
+import os
+import time
+
+
+class FileBaton:
+    '''A primitive, file-based synchronization utility.'''
+
+    def __init__(self, lock_file_path, wait_seconds=0.1):
+        '''
+        Creates a new :class:`FileBaton`.
+
+        Args:
+            lock_file_path: The path to the file used for locking.
+            wait_seconds: The seconds to periorically sleep (spin) when
+                calling ``wait()``.
+        '''
+        self.lock_file_path = lock_file_path
+        self.wait_seconds = wait_seconds
+        self.fd = None
+
+    def try_acquire(self):
+        '''
+        Tries to atomically create a file under exclusive access.
+
+        Returns:
+            True if the file could be created, else False.
+        '''
+        try:
+            self.fd = os.open(self.lock_file_path, os.O_CREAT | os.O_EXCL)
+            return True
+        except FileExistsError:
+            return False
+
+    def wait(self):
+        '''
+        Periodically sleeps for a certain amount until the baton is released.
+
+        The amount of time slept depends on the ``wait_seconds`` parameter
+        passed to the constructor.
+        '''
+        while os.path.exists(self.lock_file_path):
+            time.sleep(self.wait_seconds)
+
+    def release(self):
+        '''Releaes the baton and removes its file.'''
+        os.close(self.fd)
+        os.remove(self.lock_file_path)