* fix #541 #542

* Update train.py

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Squashed commit of the following:

commit d738487089
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 17:33:38 2020 +0700

    Adding world_size

    Reduce calls to torch.distributed. For use in create_dataloader.

commit e742dd9619
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 15:38:48 2020 +0800

    Make SyncBN a choice

commit e90d400438
Merge: 5bf8beb cd90360
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Tue Jul 14 15:32:10 2020 +0800

    Merge pull request #6 from NanoCode012/patch-5

    Update train.py

commit cd9036017e
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 13:39:29 2020 +0700

    Update train.py

    Remove redundant `opt.` prefix.

commit 5bf8bebe88
Merge: c9558a9 a1c8406
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 14:09:51 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov5 into feature/DDP_fixed

commit c9558a9b51
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 13:51:34 2020 +0800

    Add device allocation for loss compute

commit 4f08c692fb
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 9 11:16:27 2020 +0800

    Revert drop_last

commit 1dabe33a5a
Merge: a1ce9b1 4b8450b
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 9 11:15:49 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit a1ce9b1e96
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 9 11:15:21 2020 +0800

    fix lr warning

commit 4b8450b46d
Merge: b9a50ae 02c63ef
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Wed Jul 8 21:24:24 2020 +0800

    Merge pull request #4 from NanoCode012/patch-4

    Add drop_last for multi gpu

commit 02c63ef81c
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Wed Jul 8 10:08:30 2020 +0700

    Add drop_last for multi gpu

commit b9a50aed48
Merge: ec2dc6c 121d90b
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 7 19:48:04 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov5 into feature/DDP_fixed

commit ec2dc6cc56
Merge: d0326e3 82a6182
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 7 19:34:31 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit d0326e398d
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 7 19:31:24 2020 +0800

    Add SyncBN

commit 82a6182b3a
Merge: 96fa40a 050b2a5
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Tue Jul 7 19:21:01 2020 +0800

    Merge pull request #1 from NanoCode012/patch-2

    Convert BatchNorm to SyncBatchNorm

commit 050b2a5a79
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 7 12:38:14 2020 +0700

    Add cleanup for process_group

commit 2aa330139f
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 7 12:07:40 2020 +0700

    Remove apex.parallel. Use torch.nn.parallel

    For future compatibility

commit 77c8e27e60
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 7 01:54:39 2020 +0700

    Convert BatchNorm to SyncBatchNorm

commit 96fa40a3a9
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Mon Jul 6 21:53:56 2020 +0800

    Fix the datset inconsistency problem

commit 16e7c269d0
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Mon Jul 6 11:34:03 2020 +0800

    Add loss multiplication to preserver the single-process performance

commit e838055630
Merge: 625bb49 3bdea3f
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Fri Jul 3 20:56:30 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov5 into feature/DDP_fixed

commit 625bb49f4e
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 2 22:45:15 2020 +0800

    DDP established

* Squashed commit of the following:

commit 94147314e5
Merge: 65157e2 37acbdc
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 16 14:00:17 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov4 into feature/DDP_fixed

commit 37acbdc0b6
Author: Glenn Jocher <glenn.jocher@ultralytics.com>
Date:   Wed Jul 15 20:03:41 2020 -0700

    update test.py --save-txt

commit b8c2da4a0d
Author: Glenn Jocher <glenn.jocher@ultralytics.com>
Date:   Wed Jul 15 20:00:48 2020 -0700

    update test.py --save-txt

commit 65157e2fc9
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Wed Jul 15 16:44:13 2020 +0800

    Revert the README.md removal

commit 1c802bfa50
Merge: cd55b44 0f3b8bb
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Wed Jul 15 16:43:38 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit cd55b445c4
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Wed Jul 15 16:42:33 2020 +0800

    fix the DDP performance deterioration bug.

commit 0f3b8bb1fa
Author: Glenn Jocher <glenn.jocher@ultralytics.com>
Date:   Wed Jul 15 00:28:53 2020 -0700

    Delete README.md

commit f5921ba1e3
Merge: 85ab2f3 bd3fdbb
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Wed Jul 15 11:20:17 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit bd3fdbbf1b
Author: Glenn Jocher <glenn.jocher@ultralytics.com>
Date:   Tue Jul 14 18:38:20 2020 -0700

    Update README.md

commit c1a97a7767
Merge: 2bf86b8 f796708
Author: Glenn Jocher <glenn.jocher@ultralytics.com>
Date:   Tue Jul 14 18:36:53 2020 -0700

    Merge branch 'master' into feature/DDP_fixed

commit 2bf86b892fa2fd712f6530903a0d9b8533d7447a
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 22:18:15 2020 +0700

    Fixed world_size not found when called from test

commit 85ab2f38cdda28b61ad15a3a5a14c3aafb620dc8
Merge: 5a19011 c8357ad
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 22:19:58 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit 5a19011949
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 22:19:15 2020 +0800

    Add assertion for <=2 gpus DDP

commit c8357ad5b1
Merge: e742dd9 787582f
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Tue Jul 14 22:10:02 2020 +0800

    Merge pull request #8 from MagicFrogSJTU/NanoCode012-patch-1

    Modify number of dataloaders' workers

commit 787582f972
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 20:38:58 2020 +0700

    Fixed issue with single gpu not having world_size

commit 6364892528
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 19:16:15 2020 +0700

    Add assert message for clarification

    Clarify why assertion was thrown to users

commit 69364d6050
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 17:36:48 2020 +0700

    Changed number of workers check

commit d738487089
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 17:33:38 2020 +0700

    Adding world_size

    Reduce calls to torch.distributed. For use in create_dataloader.

commit e742dd9619
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 15:38:48 2020 +0800

    Make SyncBN a choice

commit e90d400438
Merge: 5bf8beb cd90360
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Tue Jul 14 15:32:10 2020 +0800

    Merge pull request #6 from NanoCode012/patch-5

    Update train.py

commit cd9036017e
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 14 13:39:29 2020 +0700

    Update train.py

    Remove redundant `opt.` prefix.

commit 5bf8bebe88
Merge: c9558a9 a1c8406
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 14:09:51 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov5 into feature/DDP_fixed

commit c9558a9b51
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 14 13:51:34 2020 +0800

    Add device allocation for loss compute

commit 4f08c692fb
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 9 11:16:27 2020 +0800

    Revert drop_last

commit 1dabe33a5a
Merge: a1ce9b1 4b8450b
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 9 11:15:49 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit a1ce9b1e96
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 9 11:15:21 2020 +0800

    fix lr warning

commit 4b8450b46d
Merge: b9a50ae 02c63ef
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Wed Jul 8 21:24:24 2020 +0800

    Merge pull request #4 from NanoCode012/patch-4

    Add drop_last for multi gpu

commit 02c63ef81c
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Wed Jul 8 10:08:30 2020 +0700

    Add drop_last for multi gpu

commit b9a50aed48
Merge: ec2dc6c 121d90b
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 7 19:48:04 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov5 into feature/DDP_fixed

commit ec2dc6cc56
Merge: d0326e3 82a6182
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 7 19:34:31 2020 +0800

    Merge branch 'feature/DDP_fixed' of https://github.com/MagicFrogSJTU/yolov5 into feature/DDP_fixed

commit d0326e398d
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Tue Jul 7 19:31:24 2020 +0800

    Add SyncBN

commit 82a6182b3a
Merge: 96fa40a 050b2a5
Author: yzchen <Chenyzsjtu@gmail.com>
Date:   Tue Jul 7 19:21:01 2020 +0800

    Merge pull request #1 from NanoCode012/patch-2

    Convert BatchNorm to SyncBatchNorm

commit 050b2a5a79
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 7 12:38:14 2020 +0700

    Add cleanup for process_group

commit 2aa330139f
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 7 12:07:40 2020 +0700

    Remove apex.parallel. Use torch.nn.parallel

    For future compatibility

commit 77c8e27e60
Author: NanoCode012 <kevinvong@rocketmail.com>
Date:   Tue Jul 7 01:54:39 2020 +0700

    Convert BatchNorm to SyncBatchNorm

commit 96fa40a3a9
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Mon Jul 6 21:53:56 2020 +0800

    Fix the datset inconsistency problem

commit 16e7c269d0
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Mon Jul 6 11:34:03 2020 +0800

    Add loss multiplication to preserver the single-process performance

commit e838055630
Merge: 625bb49 3bdea3f
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Fri Jul 3 20:56:30 2020 +0800

    Merge branch 'master' of https://github.com/ultralytics/yolov5 into feature/DDP_fixed

commit 625bb49f4e
Author: yizhi.chen <chenyzsjtu@outlook.com>
Date:   Thu Jul 2 22:45:15 2020 +0800

    DDP established

* Fixed destroy_process_group in DP mode

* Update torch_utils.py

* Update utils.py

Revert build_targets() to current master.

* Update datasets.py

* Fixed world_size attribute not found

Co-authored-by: NanoCode012 <kevinvong@rocketmail.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>