Check -gpu_ranks option to ensure saving a model (OpenNMT#1407)

* Check -gpu_ranks option to ensure saving a model
* split condition to check -gpu_ranks inconsistency

onmt/utils/parse.py

@@ -81,15 +81,24 @@ def ckpt_model_opts(cls, ckpt_opt):
     def validate_train_opts(cls, opt):
         if opt.epochs:
             raise AssertionError(
-                "-epochs is deprecated please use -train_steps.")
+                  "-epochs is deprecated please use -train_steps.")
         if opt.truncated_decoder > 0 and max(opt.accum_count) > 1:
             raise AssertionError("BPTT is not compatible with -accum > 1")
         if opt.gpuid:
-            raise AssertionError("gpuid is deprecated \
-                  see world_size and gpu_ranks")
+            raise AssertionError(
+                  "gpuid is deprecated see world_size and gpu_ranks")
         if torch.cuda.is_available() and not opt.gpu_ranks:
             logger.info("WARNING: You have a CUDA device, \
                         should run with -gpu_ranks")
+        if opt.world_size < len(opt.gpu_ranks):
+            raise AssertionError(
+                  "parameter counts of -gpu_ranks must be less or equal "
+                  "than -world_size.")
+        if opt.world_size == len(opt.gpu_ranks) and \
+                min(opt.gpu_ranks) > 0:
+            raise AssertionError(
+                  "-gpu_ranks should have master(=0) rank "
+                  "unless -world_size is greater than len(gpu_ranks).")
 
     @classmethod
     def validate_translate_opts(cls, opt):