Update type annotations in hmm inference code.

Major changes:
- Replace `jaxtyping.Int` with `dynamax.typing.IntScalar`
  - This reflects the fact that integer scalar arrays are accepted
  - `jaxtyping.[Dtype]` cannot be used directly for type checking
    instead they must be used as part of an array.
- Fix the shape of `transition_matrix`:
  - if transition_matrix has a leading timestep axis it should be of
    length T-1 not of length T.
- Add annotation indicating that `transition_matrix` is an optional argument
- Raise ValueError when neither `transition_matrix` or `transition_fn`
  provided.

dynamax/hidden_markov_model/inference.py

@@ -6,18 +6,25 @@
 from typing import Callable, Optional, Tuple, Union, NamedTuple
 from jaxtyping import Int, Float, Array
 
-from dynamax.types import Scalar
+from dynamax.types import IntScalar, Scalar
 
 _get_params = lambda x, dim, t: x[t] if x.ndim == dim + 1 else x
 
-def get_trans_mat(transition_matrix, transition_fn, t):
+def get_trans_mat(
+        transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                          Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
+        transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]],
+        t: IntScalar
+) -> Float[Array, "num_states num_states"]:
     if transition_fn is not None:
         return transition_fn(t)
-    else:
-        if transition_matrix.ndim == 3: # (T,K,K)
+    elif transition_matrix is not None:
+        if transition_matrix.ndim == 3: # (T-1,K,K)
             return transition_matrix[t]
         else:
             return transition_matrix
+    else:
+        raise ValueError("Either `transition_matrix` or `transition_fn` must be specified.")
 
 class HMMPosteriorFiltered(NamedTuple):
     r"""Simple wrapper for properties of an HMM filtering posterior.
@@ -49,8 +56,8 @@ class HMMPosterior(NamedTuple):
     predicted_probs: Float[Array, "num_timesteps num_states"]
     smoothed_probs: Float[Array, "num_timesteps num_states"]
     initial_probs: Float[Array, " num_states"]
-    trans_probs: Optional[Union[Float[Array, "num_timesteps num_states num_states"],
-                                Float[Array, "num_states num_states"]]] = None
+    trans_probs: Optional[Union[Float[Array, "num_states num_states"],
+                                Float[Array, "num_timesteps_minus_1 num_states num_states"]]] = None
 
 
 def _normalize(u: Array, axis=0, eps=1e-15):
@@ -96,10 +103,10 @@ def _predict(probs, A):
 @partial(jit, static_argnames=["transition_fn"])
 def hmm_filter(
     initial_distribution: Float[Array, " num_states"],
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]] = None
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]] = None
 ) -> HMMPosteriorFiltered:
     r"""Forwards filtering
 
@@ -143,8 +150,8 @@ def _step(carry, t):
 
 @partial(jit, static_argnames=["transition_fn"])
 def hmm_backward_filter(
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
     transition_fn: Optional[Callable[[int], Float[Array, "num_states num_states"]]]= None
 ) -> Tuple[Scalar, Float[Array, "num_timesteps num_states"]]:
@@ -190,10 +197,10 @@ def _step(carry, t):
 @partial(jit, static_argnames=["transition_fn"])
 def hmm_two_filter_smoother(
     initial_distribution: Float[Array, " num_states"],
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]]= None,
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]]= None,
     compute_trans_probs: bool = True
 ) -> HMMPosterior:
     r"""Computed the smoothed state probabilities using the two-filter
@@ -244,10 +251,10 @@ def hmm_two_filter_smoother(
 @partial(jit, static_argnames=["transition_fn"])
 def hmm_smoother(
     initial_distribution: Float[Array, " num_states"],
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]]= None,
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]]= None,
     compute_trans_probs: bool = True
 ) -> HMMPosterior:
     r"""Computed the smoothed state probabilities using a general
@@ -324,11 +331,11 @@ def _step(carry, args):
 @partial(jit, static_argnames=["transition_fn", "window_size"])
 def hmm_fixed_lag_smoother(
     initial_distribution: Float[Array, " num_states"],
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
-    window_size: Int,
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]]= None
+    window_size: IntScalar,
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]]= None
 ) -> HMMPosterior:
     r"""Compute the smoothed state probabilities using the fixed-lag smoother.
 
@@ -438,10 +445,10 @@ def compute_posterior(filtered_probs, beta):
 @partial(jit, static_argnames=["transition_fn"])
 def hmm_posterior_mode(
     initial_distribution: Float[Array, " num_states"],
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                     Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]]= None
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]]= None
 ) -> Int[Array, " num_timesteps"]:
     r"""Compute the most likely state sequence. This is called the Viterbi algorithm.
 
@@ -486,10 +493,10 @@ def _forward_pass(state, best_next_state):
 def hmm_posterior_sample(
     key: Array,
     initial_distribution: Float[Array, " num_states"],
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     log_likelihoods: Float[Array, "num_timesteps num_states"],
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]] = None
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]] = None
 ) -> Tuple[Scalar, Int[Array, " num_timesteps"]]:
     r"""Sample a latent sequence from the posterior.
 
@@ -542,6 +549,7 @@ def _compute_sum_transition_probs(
     transition_matrix: Float[Array, "num_states num_states"],
     hmm_posterior: HMMPosterior) -> Float[Array, "num_states num_states"]:
     """Compute the transition probabilities from the HMM posterior messages.
+
     Args:
         transition_matrix (_type_): _description_
         hmm_posterior (_type_): _description_
@@ -578,11 +586,13 @@ def _step(carry, args: Tuple[Array, Array, Array, Int[Array, ""]]):
 
 
 def _compute_all_transition_probs(
-    transition_matrix: Float[Array, "num_timesteps num_states num_states"],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     hmm_posterior: HMMPosterior,
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]] = None
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]] = None
     ) -> Float[Array, "num_timesteps num_states num_states"]:
     """Compute the transition probabilities from the HMM posterior messages.
+
     Args:
         transition_matrix (_type_): _description_
         hmm_posterior (_type_): _description_
@@ -600,14 +610,12 @@ def _compute_probs(t):
     return transition_probs
 
 
-# TODO: Consider alternative annotation for return type:
-#  Float[Array, "*num_timesteps num_states num_states"] I think this would allow multiple prepended dims.
-#  Float[Array, "#num_timesteps num_states num_states"] this might accept (1, sd, sd) but not (sd, sd).
+# TODO: This is a candidate for @overloading.
 def compute_transition_probs(
-    transition_matrix: Union[Float[Array, "num_timesteps num_states num_states"],
-                             Float[Array, "num_states num_states"]],
+    transition_matrix: Optional[Union[Float[Array, "num_states num_states"],
+                                      Float[Array, "num_timesteps_minus_1 num_states num_states"]]],
     hmm_posterior: HMMPosterior,
-    transition_fn: Optional[Callable[[Int], Float[Array, "num_states num_states"]]] = None
+    transition_fn: Optional[Callable[[IntScalar], Float[Array, "num_states num_states"]]] = None
 ) -> Union[Float[Array, "num_timesteps num_states num_states"],
             Float[Array, "num_states num_states"]]:
     r"""Compute the posterior marginal distributions $p(z_{t+1}, z_t \mid y_{1:T}, u_{1:T}, \theta)$.
@@ -620,8 +628,10 @@ def compute_transition_probs(
     Returns:
         array of smoothed transition probabilities.
     """
-    reduce_sum = transition_matrix is not None and transition_matrix.ndim == 2
-    if reduce_sum:
+    if transition_matrix is None and transition_fn is None:
+        raise ValueError("Either `transition_matrix` or `transition_fn` must be specified.")
+
+    if transition_matrix is not None and transition_matrix.ndim == 2:
         return _compute_sum_transition_probs(transition_matrix, hmm_posterior)
     else:
         return _compute_all_transition_probs(transition_matrix, hmm_posterior, transition_fn=transition_fn)