ONNX model has been generated with INT64 weights?

hi,

when train a classifier by using pre-trained resnet18 and then convert the pytorch to onnx,
i meet a error : " WARNING: Your ONNX model has been generated with INT64 weights".

how does the INT64 weight come up?
my pytorch version is 1.1.0 ,torchvision is 0.3.0 and python3.6.