dtype not passed in weight_initializer

[motiwari]

This line: https://github.com/roosephu/slbo/blob/master/lunzi/nn/layers.py#L20

Should be replaced with:

self.weight = Parameter(weight_initializer([in_features, out_features], dtype=tf.float32), name='weight')

It's missing the dtype parameter, which causes an exception. Will file a PR if I have time later