Simplify task value remapping API

Summary:
X-link: meta-pytorch/botorch#3146

Simplifies the get_task_value_remapping() API from 4 parameters to 2, addressing confusion reported in #3085. 

The observed_task_values parameter is removed because the parent diff (D90769576) now makes MultiTaskGP track observed/unobserved tasks internally via _observed_task_indices and _unobserved_task_indices. The default_task_value parameter is removed because the previous behavior—silently mapping unknown tasks to an arbitrary fallback—was confusing and error-prone; instead, unrecognized tasks now map to NaN, providing an explicit error sentinel with a clear warning message.

Differential Revision: D90998243

Author: Carl Hvarfner

ax/generators/torch/botorch_modular/input_constructors/outcome_transform.py

@@ -106,17 +106,10 @@ def _outcome_transform_argparse_stratified_standardize(
     dataset = assert_is_instance(dataset, MultiTaskDataset)
     if dataset.has_heterogeneous_features:
         task_feature_index = dataset.task_feature_index or -1
-        task_values = torch.arange(len(dataset.datasets), dtype=torch.long)
     else:
         task_feature_index = dataset.task_feature_index
-        task_values = dataset.X[..., dataset.task_feature_index].unique().long()
     ssd = none_throws(search_space_digest)
-    if (ssd.target_values is not None) and (
-        target_value := ssd.target_values.get(none_throws(task_feature_index))
-    ) is not None:
-        outcome_transform_options.setdefault("default_task_value", int(target_value))
     outcome_transform_options.setdefault("stratification_idx", task_feature_index)
-    outcome_transform_options.setdefault("observed_task_values", task_values)
     outcome_transform_options.setdefault(
         "all_task_values",
         torch.tensor(