make lr finder error message more helpful.

ignite/handlers/lr_finder.py

@@ -161,13 +161,19 @@ def _log_lr_and_loss(self, trainer: Engine, output_transform: Callable, smooth_f
                     raise ValueError(
                         "if output of the engine is torch.Tensor, then "
                         "it must be 0d torch.Tensor or 1d torch.Tensor with 1 element, "
-                        f"but got torch.Tensor of shape {loss.shape}"
+                        f"but got torch.Tensor of shape {loss.shape}."
                     )
             else:
                 raise TypeError(
                     "output of the engine should be of type float or 0d torch.Tensor "
                     "or 1d torch.Tensor with 1 element, "
                     f"but got output of type {type(loss).__name__}"
+                    "You may wish to use the output_transform kwarg with the attach method e.g.\n"
+                    """
+                    lr_finder = FastaiLRFinder()
+                    with lr_finder.attach(trainer, output_transform=lambda x:x["train_loss"]) as trainer_with_lr_finder:
+                        trainer_with_lr_finder.run(dataloader_train)
+                    """
                 )
         loss = idist.all_reduce(loss)
         lr = self._lr_schedule.get_param()