Fix train_loader not initalized properly (#981)

* fix

* fix typo

* typo

src/super_gradients/training/sg_trainer/sg_trainer.py

@@ -133,6 +133,7 @@ def __init__(self, experiment_name: str, device: str = None, multi_gpu: Union[Mu
 
         # SET THE EMPTY PROPERTIES
         self.net, self.architecture, self.arch_params, self.dataset_interface = None, None, None, None
+        self.train_loader, self.valid_loader = None, None
         self.ema = None
         self.ema_model = None
         self.sg_logger = None
@@ -996,8 +997,15 @@ def forward(self, inputs, targets):
         global logger
         if training_params is None:
             training_params = dict()
-        self.train_loader = train_loader or self.train_loader
-        self.valid_loader = valid_loader or self.valid_loader
+
+        self.train_loader = train_loader if train_loader is not None else self.train_loader
+        self.valid_loader = valid_loader if valid_loader is not None else self.valid_loader
+
+        if self.train_loader is None:
+            raise ValueError("No `train_loader` found. Please provide a value for `train_loader`")
+
+        if self.valid_loader is None:
+            raise ValueError("No `valid_loader` found. Please provide a value for `valid_loader`")
 
         if hasattr(self.train_loader, "batch_sampler") and self.train_loader.batch_sampler is not None:
             batch_size = self.train_loader.batch_sampler.batch_size