FIX posible directions for LSTM and GRU

Author: Enrico Lupi

hls4ml/model/layers.py

@@ -1353,7 +1353,7 @@ class LSTM(Layer):
         Attribute('return_sequences', value_type=bool, default=False),
         Attribute('return_state', value_type=bool, default=False),
         Attribute('pass_initial_states', value_type=bool, default=False),
-        ChoiceAttribute('direction', ['forward', 'backward', 'bidirectional'], configurable=False, default='forward'),
+        ChoiceAttribute('direction', ['forward', 'backward'], configurable=False, default='forward'),
         Attribute('time_major', value_type=bool, default=False),
         WeightAttribute('weight'),
         WeightAttribute('bias'),
@@ -1410,7 +1410,7 @@ class GRU(Layer):
         Attribute('return_sequences', value_type=bool, default=False),
         Attribute('return_state', value_type=bool, default=False),
         Attribute('pass_initial_states', value_type=bool, default=False),
-        ChoiceAttribute('direction', ['forward', 'backward', 'bidirectional'], configurable=False, default='forward'),
+        ChoiceAttribute('direction', ['forward', 'backward'], configurable=False, default='forward'),
         Attribute('time_major', value_type=bool, default=False),
         ChoiceAttribute('apply_reset_gate', ['before', 'after'], configurable=False, default='after'),
         WeightAttribute('weight'),