Added atomic checkpoint creation (#689)

* Added atomic checkpoint creation

* Added documentation for _atomic_checkpoint

pytorch_lightning/trainer/training_io.py

@@ -257,17 +257,34 @@ class TrainerIOMixin(ABC):
     # --------------------
     # MODEL SAVE CHECKPOINT
     # --------------------
+    def _atomic_save(self, checkpoint, filepath):
+        """Saves a checkpoint atomically, avoiding the creation of incomplete checkpoints.
+
+        This will create a temporary checkpoint with a suffix of ``.part``, then copy it to the final location once
+        saving is finished.
+
+        Args:
+            checkpoint (object): The object to save.
+                Built to be used with the ``dump_checkpoint`` method, but can deal with anything which ``torch.save``
+                accepts.
+            filepath (str|pathlib.Path): The path to which the checkpoint will be saved.
+                This points to the file that the checkpoint will be stored in.
+        """
+        tmp_path = str(filepath) + ".part"
+        torch.save(checkpoint, tmp_path)
+        os.replace(tmp_path, filepath)
+
     def save_checkpoint(self, filepath):
         checkpoint = self.dump_checkpoint()
 
         # do the actual save
         try:
-            torch.save(checkpoint, filepath)
+            self._atomic_save(checkpoint, filepath)
         except AttributeError:
             if 'hparams' in checkpoint:
                 del checkpoint['hparams']
 
-            torch.save(checkpoint, filepath)
+            self._atomic_save(checkpoint, filepath)
 
     def restore(self, checkpoint_path, on_gpu):
 
@@ -415,12 +432,12 @@ class TrainerIOMixin(ABC):
         # do the actual save
         # TODO: fix for anything with multiprocess DP, DDP, DDP2
         try:
-            torch.save(checkpoint, filepath)
+            self._atomic_save(checkpoint, filepath)
         except AttributeError:
             if 'hparams' in checkpoint:
                 del checkpoint['hparams']
 
-            torch.save(checkpoint, filepath)
+            self._atomic_save(checkpoint, filepath)
 
         return filepath