ADLR/megatron-lm!2136 - tests: Fix GPT test

Author: ko3n1g

tests/functional_tests/test_cases/gpt/gpt3_15b_8t_release/model_config.yaml

@@ -25,7 +25,7 @@ MODEL_ARGS:
   --micro-batch-size: 4
   --rampup-batch-size: "384 384 97656250"
   --global-batch-size: 1152
-  --train-samples: 4882812
+  --train-samples: 19531250
   --manual-gc: true
 
   # Transformer Engine args