Fix off-by-one error in split_by_batchsize

If the batch size is equal to the number of parameters,
we want _split_by_batchsize to return a single batch and
not two.

Update batch_size calculation to make sure the new
_split_by_batchsize works when the number of parameters
is less than the hard-coded batch_size of 1000.

src/ert/analysis/_es_update.py

@@ -421,7 +421,31 @@ def _load_observations_and_responses(
 def _split_by_batchsize(
     arr: npt.NDArray[np.int_], batch_size: int
 ) -> List[npt.NDArray[np.int_]]:
-    return np.array_split(arr, int((arr.shape[0] / batch_size)) + 1)
+    """
+    Splits an array into sub-arrays of a specified batch size.
+
+    Examples
+    --------
+    >>> num_params = 10
+    >>> batch_size = 3
+    >>> s = np.arange(0, num_params)
+    >>> _split_by_batchsize(s, batch_size)
+    [array([0, 1, 2, 3]), array([4, 5, 6]), array([7, 8, 9])]
+
+    >>> num_params = 10
+    >>> batch_size = 10
+    >>> s = np.arange(0, num_params)
+    >>> _split_by_batchsize(s, batch_size)
+    [array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])]
+
+    >>> num_params = 10
+    >>> batch_size = 20
+    >>> s = np.arange(0, num_params)
+    >>> _split_by_batchsize(s, batch_size)
+    [array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])]
+    """
+    sections = 1 if batch_size > len(arr) else len(arr) // batch_size
+    return np.array_split(arr, sections)
 
 
 def _update_with_row_scaling(
@@ -535,7 +559,6 @@ def analysis_ES(
         truncation = module.enkf_truncation
 
         if module.localization:
-            batch_size: int = 1000
             smoother_adaptive_es = AdaptiveESMDA(
                 covariance=observation_errors**2,
                 observations=observation_values,
@@ -570,6 +593,7 @@ def analysis_ES(
             )
             if module.localization:
                 num_params = temp_storage[param_group.name].shape[0]
+                batch_size = min(1000, num_params)
                 batches = _split_by_batchsize(np.arange(0, num_params), batch_size)
 
                 progress_callback(