Address KFAC refactor suggestions

curvlinops/kfac.py

@@ -261,7 +261,7 @@ def __init__(
                 "Only mc_samples=1 is supported for `fisher_type != FisherType.MC`."
             )
         if fisher_type == FisherType.FORWARD_ONLY and correct_eigenvalues:
-            raise ValueError(
+            raise NotImplementedError(
                 "Correcting eigenvalues is not supported for FisherType.FORWARD_ONLY."
             )
         if kfac_approx not in self._SUPPORTED_KFAC_APPROX:
@@ -277,6 +277,8 @@ def __init__(
         self._mc_samples = mc_samples
         self._kfac_approx = kfac_approx
         self._correct_eigenvalues = correct_eigenvalues
+        # Initialize flag which determines whether to compute the KFAC factors or the
+        # eigenvalue correction in the forward-backward pass(es)
         self._compute_eigenvalue_correction_flag = False
         self._input_covariances: Dict[str, Tensor] = {}
         self._gradient_covariances: Dict[str, Tensor] = {}
@@ -457,7 +459,9 @@ def _left_and_right_multiply(
             aaT: Input covariance Kronecker factor or its eigenvectors. ``None`` for
                 biases.
             ggT: Gradient covariance Kronecker factor or its eigenvectors.
-            eigenvalues: Corrected eigenvalues for the EKFAC approximation.
+            eigenvalues: Eigenvalues of the (E)KFAC approximation when multiplying with
+                the eigendecomposition of the KFAC approximation. ``None`` for the
+                non-decomposed KFAC approximation.
 
         Returns:
             Matrix-multiplication result ``KFAC @ M_joint``.
@@ -473,7 +477,7 @@ def _left_and_right_multiply(
             M_joint = einsum(
                 ggT_eigvecs, M_joint, aaT_eigvecs, "i j, m i k, k l -> m j l"
             )
-            # Multiply by eigenvalues.
+            # Multiply (broadcasted) by eigenvalues.
             M_joint.mul_(eigenvalues)
             # Transform back to standard basis.
             M_joint = einsum(
@@ -497,7 +501,9 @@ def _separate_left_and_right_multiply(
             aaT: Input covariance Kronecker factor or its eigenvectors. ``None`` for
                 biases.
             ggT: Gradient covariance Kronecker factor or its eigenvectors.
-            eigenvalues: Corrected eigenvalues for the EKFAC approximation.
+            eigenvalues: Eigenvalues of the (E)KFAC approximation when multiplying with
+                the eigendecomposition of the KFAC approximation. ``None`` for the
+                non-decomposed KFAC approximation.
 
         Returns:
             Matrix-multiplication result ``KFAC @ M_torch``.
@@ -510,12 +516,13 @@ def _separate_left_and_right_multiply(
                 # If `eigenvalues` is not `None`, we transform to eigenbasis here
                 M_torch[pos] = einsum(M_w, aaT, "m i j, j k -> m i k")
 
-            dims = "m j ... -> m i ..." if eigenvalues is None else "m i ... -> m j ..."
-            # If `eigenvalues` is not `None`, we transform to eigenbasis here
-            M_torch[pos] = einsum(ggT, M_torch[pos], f"i j, {dims}")
+            # If `eigenvalues` is not `None`, we convert to eigenbasis here
+            M_torch[pos] = einsum(
+                ggT.T if eigenvalues else ggT, M_torch[pos], "i j, m j ... -> m i ..."
+            )
 
             if eigenvalues is not None:
-                # Multiply by eigenvalues and transform back to standard basis
+                # Multiply (broadcasted) by eigenvalues, convert back to original basis
                 M_torch[pos].mul_(eigenvalues[pos])
                 if p_name == "weight":
                     M_torch[pos] = einsum(M_torch[pos], aaT, "m i j, k j -> m i k")
@@ -1101,7 +1108,10 @@ def _set_or_add_(
         elif isinstance(dictionary[key], Tensor) and isinstance(value, Tensor):
             dictionary[key].add_(value)
         else:
-            raise ValueError("Incompatible types for addition.")
+            raise ValueError(
+                "Incompatible types for addition: dictionary value of type "
+                f"{type(dictionary[key])} and value to be added of type {type(value)}."
+            )
         return dictionary
 
     @classmethod