[refactor results 1] - refactor backward (#2276)

* move backward

* refactor backward to remove 16 bit from user override

* refactor backward to remove 16 bit from user override

* Update pytorch_lightning/core/hooks.py

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

pytorch_lightning/core/hooks.py

@@ -182,26 +182,20 @@ def backward(self, trainer, loss: Tensor, optimizer: Optimizer, optimizer_idx: i
 
         Example::
 
-            def backward(self, use_amp, loss, optimizer):
-                if use_amp:
-                    with amp.scale_loss(loss, optimizer) as scaled_loss:
-                        scaled_loss.backward()
-                else:
-                    loss.backward()
+            def backward(self, trainer, loss, optimizer, optimizer_idx):
+                loss.backward()
 
         """
-        if trainer.precision == 16:
-            # .backward is not special on 16-bit with TPUs
-            if trainer.on_tpu:
-                return
+        loss.backward()
+
+    def amp_scale_loss(self, unscaled_loss, optimizer, optimizer_idx):
+        if self.trainer.use_native_amp:
+            scaled_loss = self.trainer.scaler.scale(unscaled_loss)
 
-            if self.trainer.use_native_amp:
-                self.trainer.scaler.scale(loss).backward()
-            else:
-                with amp.scale_loss(loss, optimizer) as scaled_loss:
-                    scaled_loss.backward()
         else:
-            loss.backward()
+            scaled_loss = amp.scale_loss(unscaled_loss, optimizer)
+
+        return scaled_loss
 
     def transfer_batch_to_device(self, batch: Any, device: torch.device) -> Any:
         """

pytorch_lightning/trainer/training_loop.py

@@ -609,6 +609,11 @@ def optimizer_closure():
                     # backward pass
                     model_ref = self.get_model()
                     with self.profiler.profile('model_backward'):
+                        # scale loss for 16 bit
+                        if self.precision == 16 and not self.on_tpu:
+                            closure_loss = model_ref.amp_scale_loss(closure_loss, optimizer, opt_idx)
+
+                        # do backward pass
                         model_ref.backward(self, closure_loss, optimizer, opt_idx)
 
                     # track metrics for callbacks