add --max-tokens-valid option for validation

Summary: Add the max-token-valid option. Sometime a separate max batch tokens for validation may be helpful, for example when there is a long sequence in validation set thats larger than max_tokens (it's rare in MT but could happen in ASR or AST).

Reviewed By: myleott

Differential Revision: D16076951

fbshipit-source-id: ae7f4218594580b9450a8196d7afa1e7e2018aee

train.py

@@ -195,7 +195,7 @@ def validate(args, trainer, task, epoch_itr, subsets):
         # Initialize data iterator
         itr = task.get_batch_iterator(
             dataset=task.dataset(subset),
-            max_tokens=args.max_tokens,
+            max_tokens=args.max_tokens_valid,
             max_sentences=args.max_sentences_valid,
             max_positions=utils.resolve_max_positions(
                 task.max_positions(),