Values:
MODEL_TRAINING_MODE_UNSPECIFIED (0):
Not specified. MODEL_TRAINING_MODE_AUTOMATIC will be
used.
MODEL_TRAINING_MODE_AUTOMATIC (1):
NLU model training is automatically triggered
when a flow gets modified. User can also
manually trigger model training in this mode.
MODEL_TRAINING_MODE_MANUAL (2):
User needs to manually trigger NLU model
training. Best for large flows whose models take
long time to train.