Got stuck when running estimator api example

[hkvision]

I'm using cpu to run the code here without any code change: https://mxnet.apache.org/api/python/docs/tutorials/packages/gluon/training/fit_api_tutorial.html
But it got stuck after printing out the following:

Training begin: using optimizer SGD with current learning rate 0.0400 
Train for 2 epochs.
[Epoch 0] Begin, current learning rate: 0.0400

What could be the problem?

Also I want to ask is Estimator API officially recommended? Coz except this one, I cannot find any other one elsewhere...
And estimator.fit only accepts dataloaders, wondering how can I transform ImageIter to DataLoader? Still I can't find related documentation anywhere...

Thanks so much in advance!