reorgnize colotensor directory (#1062)

* reorgnize colotensor directory

* polish code

colossalai/nn/_ops/_utils.py

@@ -0,0 +1,12 @@
+import torch
+from typing import Union, Optional
+from colossalai.tensor import ColoTensor
+
+GeneralTensor = Union[ColoTensor, torch.Tensor]
+Number = Union[int, float]
+
+
+def convert_to_colo_tensor(tensor: Optional[GeneralTensor]) -> Optional[ColoTensor]:
+    if tensor is not None and not isinstance(tensor, ColoTensor):
+        tensor = ColoTensor.from_torch_tensor(tensor)
+    return tensor