Add safety checks for 'data' key in MegatronGPTModel cfg (NVIDIA#8991)

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py

@@ -367,9 +367,10 @@ def __init__(self, cfg: DictConfig, trainer: Trainer):
         self.log_train_loss = bool(int(os.getenv("NEMO_LOG_TRAIN_LOSS", 1)))
         self.log_memory_usage = bool(int(os.getenv("NEMO_LOG_MEMORY_USAGE", 0)))
         self.loss_broadcast_src_rank = None
-        self.return_output_tensors = cfg.data.get('return_output_tensors', False)
-        self.validation_drop_last = cfg.data.get('validation_drop_last', True)
-        self.sample_weight = cfg.data.get('sample_weight', 'token')
+        data_cfg = cfg.get('data', {})
+        self.return_output_tensors = data_cfg.get('return_output_tensors', False)
+        self.validation_drop_last = data_cfg.get('validation_drop_last', True)
+        self.sample_weight = data_cfg.get('sample_weight', 'token')
         self.validation_param_sync_overlap = self.cfg.get('validation_param_sync_overlap', False)
 
         self.inference_params = None