[fix] fix optim bwd;

colossalai/interface/optimizer.py

@@ -58,7 +58,7 @@ class OptimizerWrapper:
     # def backward_by_grad(self, tensor: Tensor, grad: Tensor):
     #     torch.autograd.backward(tensor, grad)
 
-    def backward_by_grad(self, tensor: Tensor, grad: Tensor, inputs: Tensor, retain_graph: bool = False):
+    def backward_by_grad(self, tensor: Tensor, grad: Tensor, inputs: Tensor = None, retain_graph: bool = False):
         """
         Performs a backward pass for dx or dw,
         for dx, we only calculate dx = w*dy here