[chore][benchmarks] Add license file headers for all files in fairscale/benchmarks (#670)

* add license file headers for all files

* fix lint

benchmarks/datasets/mnist.py

+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
+
 import logging
 from pathlib import Path
 import shutil

benchmarks/datasets/wikitext2_data.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 import io
 import tempfile

benchmarks/experimental/benchmark_dataset.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 import torch
 from torch.utils.data import Dataset

benchmarks/golden_configs/lm_wikitext2.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 import torch.nn as nn
 

benchmarks/golden_configs/oss_mnist.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 
 def get_golden_real_stats():

benchmarks/models/transformer_lm.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 import math
 

benchmarks/oss.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
-
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 import argparse
 from enum import Enum

benchmarks/pipe.py

-# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
 
 import argparse
 from collections import defaultdict