Set drop_prob = 0 for causal models (#125)

*Description of changes:* This PR sets `drop_prob = 0` when training
causal models. Missing values are problematic for causal model training.


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scripts/training/train.py

@@ -320,7 +320,7 @@ def __init__(
         self.tokenizer = tokenizer
         self.context_length = context_length
         self.prediction_length = prediction_length
-        self.drop_prob = drop_prob
+        self.drop_prob = drop_prob if model_type == "seq2seq" else 0.0
         self.min_past = min_past or prediction_length
         self.model_type = model_type
         self.imputation_method = imputation_method or LeavesMissingValues()