tf.lbeta(x, name='lbeta')
Computes ln(|Beta(x)|), reducing along the last dimension.
Given one-dimensional z = [z_0,...,z_{K-1}], we define
Beta(z) = \prod_j Gamma(z_j) / Gamma(\sum_j z_j)
And for n + 1 dimensional x with shape [N1, ..., Nn, K], we define lbeta(x)[i1, ..., in] = Log(|Beta(x[i1, ..., in, :])|). In other words, the last dimension is treated as the z vector.
Note that if z = [u, v], then Beta(z) = int_0^1 t^{u-1} (1 - t)^{v-1} dt, which defines the traditional bivariate beta func