Fix gradients when using AMP (#70)

retain grad related attrs while casting
Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>

transformer_engine/pytorch/utils.py

@@ -169,4 +169,5 @@ def safely_set_viewless_tensor_data(
 
 def cast_if_needed(tensor: torch.Tensor, dtype: torch.dtype) -> torch.Tensor:
     """Cast tensor to dtype"""
+    with torch.enable_grad():
         return tensor if tensor is None or tensor.dtype == dtype else tensor.to(dtype)