set zero3 optimizer betas to auto so they inherit from HF trainer config (#507)

deepspeed/zero3.json

@@ -35,10 +35,7 @@
     "type": "AdamW",
     "params": {
       "lr": "auto",
-      "betas": [
-        0.9,
-        0.95
-      ],
+      "betas": "auto",
       "eps": 1e-8,
       "weight_decay": "auto"
     }