Simplify search flag logic (#947)

Use net search flag only as default fall back
when search option is not explicitly provided for ChoiceLayer.
DecideLayer and co will always apply.

ChoiceLayer, new add_to_beam_scores option
to control whether the score should be added to the beam
when not doing search.

Fix #946.

Related:
rwth-i6/returnn_common#18

Author: albertz

returnn/tf/layers/basic.py

@@ -10684,7 +10684,7 @@ def get_value(self):
     """
     assert self.layer
     search_choices = self.layer.get_search_choices()
-    assert self.layer.network.search_flag and search_choices, "no search?"
+    assert search_choices, "no search?"
     # Negative score, because we minimize the loss, i.e. maximize the score.
     return self.reduce_func(-search_choices.beam_scores)