Add dummy task for translation benchmarking (#1212)

Summary: Pull Request resolved: fairinternal/fairseq-py#1212

Test Plan:
python train.py \
    -a transformer \
    --clip-norm 0.4 --optimizer adam --lr 0.001 \
    --dropout 0.0 \
    --decoder-layers 7 \
    --encoder-layers 7 \
    --encoder-ffn-embed-dim 2048 \
    --decoder-ffn-embed-dim 2048 \
    --encoder-embed-dim 1024 \
    --decoder-embed-dim 1024 \
    --max-tokens 8192 \
    --criterion cross_entropy --max-update 50 \
    --attention-dropout 0.0 \
    --adam-betas '(0.9, 0.98)' \
    --disable-validation --no-save \
    --task dummy_mt

# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes # (issue).

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �

Reviewed By: myleott

Differential Revision: D22484873

Pulled By: msbaines

fbshipit-source-id: bc61165ab91290d0b6aa2077c968ab537bce8a6a

Author: Mandeep Baines

fairseq/benchmark/__init__.py

@@ -8,4 +8,5 @@
     dummy_lm,
     dummy_masked_lm,
     dummy_model,
+    dummy_mt,
 )

fairseq/benchmark/dummy_mt.py

@@ -0,0 +1,120 @@
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+import logging
+
+import numpy as np
+import torch
+
+from fairseq.data import Dictionary, FairseqDataset
+from fairseq.tasks import FairseqTask, register_task
+
+
+logger = logging.getLogger(__name__)
+
+
+@register_task('dummy_mt')
+class DummyMTTask(FairseqTask):
+
+    @staticmethod
+    def add_args(parser):
+        """Add task-specific arguments to the parser."""
+        parser.add_argument('--dict-size', default=49996, type=int)
+        parser.add_argument('--dataset-size', default=100000, type=int)
+        parser.add_argument('--tokens-per-sample', default=512, type=int,
+                            help='max number of total tokens over all segments '
+                                 'per sample for BERT dataset')
+
+    def __init__(self, args, dictionary):
+        super().__init__(args)
+        self.dictionary = dictionary
+        self.seed = args.seed
+
+        dictionary.pad_to_multiple_(8)  # often faster if divisible by 8
+
+        seq = torch.arange(args.tokens_per_sample + 1) + dictionary.pad() + 1
+
+        self.dummy_src = seq[:-1]
+        self.dummy_tgt = seq[1:]
+
+    @classmethod
+    def setup_task(cls, args, **kwargs):
+        """Setup the task. """
+        dictionary = Dictionary()
+        for i in range(args.dict_size):
+            dictionary.add_symbol('word{}'.format(i))
+        logger.info('dictionary: {} types'.format(len(dictionary)))
+        return cls(args, dictionary)
+
+    def load_dataset(self, split, epoch=1, combine=False, **kwargs):
+        """Load a given dataset split.
+        Args:
+            split (str): name of the split (e.g., train, valid, test)
+        """
+        if self.args.max_sentences is not None:
+            bsz = self.args.max_sentences
+        else:
+            bsz = max(1, self.args.max_tokens // self.args.tokens_per_sample)
+        tgt = torch.stack([self.dummy_tgt for _ in range(bsz)])
+        self.datasets[split] = DummyDataset(
+            {
+                'id': 1,
+                'net_input': {
+                    'src_tokens': torch.stack([self.dummy_src for _ in range(bsz)]),
+                    'src_lengths': torch.full(
+                        (bsz, ), self.args.tokens_per_sample, dtype=torch.long
+                    ),
+                    'prev_output_tokens': tgt.clone(),
+                },
+                'target': tgt,
+                'nsentences': bsz,
+                'ntokens': bsz * self.args.tokens_per_sample,
+            },
+            num_items=self.args.dataset_size,
+            item_size=self.args.tokens_per_sample,
+        )
+
+    @property
+    def source_dictionary(self):
+        return self.dictionary
+
+    @property
+    def target_dictionary(self):
+        return self.dictionary
+
+
+class DummyDataset(FairseqDataset):
+
+    def __init__(self, batch, num_items, item_size):
+        super().__init__()
+        self.batch = batch
+        self.num_items = num_items
+        self.item_size = item_size
+
+    def __getitem__(self, index):
+        return index
+
+    def __len__(self):
+        return self.num_items
+
+    def collater(self, samples):
+        return self.batch
+
+    @property
+    def sizes(self):
+        return np.array([self.item_size] * self.num_items)
+
+    def num_tokens(self, index):
+        return self.item_size
+
+    def size(self, index):
+        return self.item_size
+
+    def ordered_indices(self):
+        return np.arange(self.num_items)
+
+    @property
+    def supports_prefetch(self):
+        return False