[BUG] Server's sampler cannot process received features from client side

[ZSK_UST]

Describe the bug
In the server/mistnet.py, the sampler class should be initiated with datasets who has the method num_train_examples(). After I check the code, the current version is passed the list of features directly to sample which causes the error.

To Reproduce
./run --config=configs/MNIST/mistnet_lenet5.yml

Screenshots