Fix trainer.fit_loop.split_idx reference (#8601)

* Fix split idx reference

* Update CHANGELOG

* Add comment

pytorch_lightning/loops/fit_loop.py

@@ -74,7 +74,7 @@ def batch_idx(self) -> int:
     @property
     def split_idx(self) -> int:
         """Returns the index of the current batch split (within the current batch) for bptt"""
-        return self.epoch_loop.split_idx
+        return self.epoch_loop.batch_loop.split_idx
 
     @property
     def min_steps(self) -> int:

pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py

@@ -241,9 +241,9 @@ def on_batch_start(self) -> None:
         self._epoch_end_reached = False
 
     def epoch_end_reached(self):
-        self.trainer.logger_connector._epoch_end_reached = True
-        self.trainer.logger_connector._batch_idx = None
-        self.trainer.logger_connector._split_idx = None
+        self._epoch_end_reached = True
+        self._batch_idx = None
+        self._split_idx = None
 
     def on_epoch_end(self) -> None:
         assert self._epoch_end_reached

tests/trainer/logging_/test_train_loop_logging.py

@@ -214,62 +214,62 @@ def validation_step(self, batch, batch_idx):
 
 
 def test_tbptt_log(tmpdir):
-    """
-    Tests that only training_step can be used
-    """
     truncated_bptt_steps = 2
-    sequence_size = 30
-    batch_size = 30
-
-    x_seq = torch.rand(batch_size, sequence_size, 1)
-    y_seq_list = torch.rand(batch_size, sequence_size, 1).tolist()
+    N, T, F = 32, 15, 1  # batches x timesteps (sequence size) x features
+    batch_size = 10
+    assert T % truncated_bptt_steps != 0, "Should test leftover time steps"
 
     class MockSeq2SeqDataset(torch.utils.data.Dataset):
-        def __getitem__(self, i):
-            return x_seq, y_seq_list
+        def __init__(self):
+            self.x_seq = torch.randn(N, T, F)
+            self.y_seq = torch.randn(N, T, F)
+
+        def __getitem__(self, index):
+            return self.x_seq[index], self.y_seq[index]
 
         def __len__(self):
-            return 1
+            return N
 
     class TestModel(BoringModel):
         def __init__(self):
             super().__init__()
             self.test_hidden = None
-            self.layer = torch.nn.Linear(2, 2)
+            self.layer = torch.nn.LSTM(input_size=F, hidden_size=T, batch_first=True)
 
         def training_step(self, batch, batch_idx, hiddens):
             assert hiddens == self.test_hidden, "Hidden state not persistent between tbptt steps"
             if hiddens is not None:
                 assert hiddens.grad_fn is None
-            self.test_hidden = torch.tensor(2.0, requires_grad=True).pow(2)
+            split_idx = self.trainer.fit_loop.split_idx
+            self.test_hidden = torch.tensor(split_idx, requires_grad=True, dtype=torch.float).pow(2)
 
-            x_tensor, y_list = batch
-            assert x_tensor.shape[1] == truncated_bptt_steps, "tbptt split Tensor failed"
+            x, y = batch
+            if self.trainer.fit_loop.epoch_loop.batch_loop.done:
+                # last split idx, not aligned
+                assert x.shape[1] == T % truncated_bptt_steps
+                assert y.shape[1] == T % truncated_bptt_steps
+            else:
+                assert x.shape[1] == truncated_bptt_steps
+                assert y.shape[1] == truncated_bptt_steps
 
-            y_tensor = torch.tensor(y_list, dtype=x_tensor.dtype)
-            assert y_tensor.shape[1] == truncated_bptt_steps, "tbptt split list failed"
-
-            pred = self(x_tensor.view(batch_size, truncated_bptt_steps))
-            loss = torch.nn.functional.mse_loss(pred, y_tensor.view(batch_size, truncated_bptt_steps))
+            pred, _ = self(x)
+            loss = torch.nn.functional.mse_loss(pred, y)
 
             self.log("a", loss, on_epoch=True)
 
             return {"loss": loss, "hiddens": self.test_hidden}
 
-        def on_train_epoch_start(self) -> None:
+        def on_train_batch_start(self, *args, **kwargs) -> None:
             self.test_hidden = None
 
         def train_dataloader(self):
-            return torch.utils.data.DataLoader(
-                dataset=MockSeq2SeqDataset(), batch_size=batch_size, shuffle=False, sampler=None
-            )
+            return torch.utils.data.DataLoader(dataset=MockSeq2SeqDataset(), batch_size=batch_size)
 
     model = TestModel()
     model.training_epoch_end = None
 
     trainer = Trainer(
         default_root_dir=tmpdir,
-        limit_train_batches=10,
         limit_val_batches=0,
         truncated_bptt_steps=truncated_bptt_steps,
         max_epochs=2,
@@ -278,6 +278,8 @@ def train_dataloader(self):
     )
     trainer.fit(model)
 
+    assert trainer.fit_loop.batch_idx == N // batch_size
+    assert trainer.fit_loop.split_idx == T // truncated_bptt_steps
     assert set(trainer.logged_metrics) == {"a_step", "a_epoch", "epoch"}