Fix race condition when creating build dir (pytorch#30956)

Summary:
The original `check-and-act` style can raise `FileExistsError` when multiple processes are jit-compiling the extension on the same node.
Pull Request resolved: pytorch#30956

Differential Revision: D19262570

Pulled By: ezyang

fbshipit-source-id: bb18c72e42648770b47f9378ac7c3929c3c03efc

torch/utils/cpp_extension.py

@@ -1048,11 +1048,13 @@ def _get_build_directory(name, verbose):
             root_extensions_directory))
 
     build_directory = os.path.join(root_extensions_directory, name)
-    if not os.path.exists(build_directory):
+    try:
+        os.makedirs(build_directory)
         if verbose:
             print('Creating extension directory {}...'.format(build_directory))
-        # This is like mkdir -p, i.e. will also create parent directories.
-        os.makedirs(build_directory)
+    except FileExistsError:
+        if verbose:
+            print('Using existing directory {}...'.format(build_directory))
 
     return build_directory