cleanup HMM SGD code

[murphyk]

Rename hmm_fit_minibatch_gradient_descent in
https://github.com/probml/ssm-jax/blob/main/ssm_jax/hmm/learning.py#L75
to be hmm_fit_sgd.
Rename emissions to be batch_emissions. Add commetn that input is (N,T)
but you take a minibatch of size (B,T) at each step.

Remove old hmm_fit_sgd.

Move permutation step in
https://github.com/probml/ssm-jax/blob/main/ssm_jax/hmm/learning.py#L93
inside of _sample_minibatches. Split the RNG key.
(Also check if B=N no need to do random permutation)
Add comment that you are sampling a random susbet of entire sequence, not time steps.