Change a typo num_samples -> num_sample so that make sampling work

[TheaperDeng]

In model.py there are two typos for each run_{} function.

    enc1.num_samples = 10
    enc2.num_samples = 25

should be changed to

    enc1.num_sample = 10
    enc2.num_sample = 25

because the encoder class has variable self.num_sample