update configs

config/ppo/Walker.yaml

     trainer: ppo
     batch_size: 2048
     beta: 0.005
-    buffer_size: 204800
+    buffer_size: 20480
     epsilon: 0.2
     hidden_units: 512
     lambd: 0.95

config/ppo/WalkerDynamic.yaml

     trainer: ppo
     batch_size: 2048
     beta: 0.005
-    buffer_size: 409600
+    buffer_size: 20480
     epsilon: 0.2
     hidden_units: 512
     lambd: 0.95