Functionally is torch.multinomial the same as torch.distributions.categorical.Categorical?

For example, if I provide a probability array of [0.5, 0.5], both functions will sample the index [0,1] with equal probability?

Yes:

[torch.distributions.categorical.Categorical()] is equivalent to the distribution that torch.multinomial() samples from.

• https://pytorch.org/docs/stable/distributions.html#categorical