Add gmm and Update gaussian

[sbsekiguchi]

Added gmm and gaussian of the numpy models.
In addition, updated the gaussian distribution's API.

The API change is like following:

Previous :

batch_size = 10
output_dim = 10
input_shape = (batch_size, output_dim)
mean = np.zeros(shape=input_shape)
sigma = np.ones(shape=input_shape) * 5.
ln_var = np.log(sigma) * 2.
distribution = D.Gaussian(mean, ln_var)
# return nn.Variable
assert isinstance(distribution.sample(), nn.Variable)

Updated:

batch_size = 10
output_dim = 10
input_shape = (batch_size, output_dim)
mean = np.zeros(shape=input_shape)
sigma = np.ones(shape=input_shape) * 5.
ln_var = np.log(sigma) * 2.
# You have to pass the nn.Variable if you want to get nn.Variable as all class method's return.
distribution = D.Gaussian(nn.Variable.from_numpy_array(mean), nn.Variable.from_numpy_array(ln_var))
assert isinstance(distribution.sample(), nn.Variable)

# If you pass np.ndarray, then all class methods return np.ndarray
# Currently, only support without batch shape (i.e. mean.shape = (dims,), ln_var.shape = (dims, dims)).
distribution = D.Gaussian(mean[0], np.diag(ln_var[0]))  # without batch
assert isinstance(distribution.sample(), np.ndarray)