Put Time Horizon back into the default training config for BC (#1291)

Put Time Horizon back into the default training config for BC

config/bc_config.yaml

     max_steps: 5.0e4
     memory_size: 256
     batches_per_epoch: 10
+    time_horizon: 64
     num_epoch: 5
     num_layers: 2
     summary_freq: 1000