fix torch.nonzero usage in pytorch 1.5 (#2602)

mmdet/core/bbox/assigners/atss_assigner.py

mmdet/core/bbox/assigners/center_region_assigner.py

mmdet/core/bbox/assigners/max_iou_assigner.py

mmdet/core/bbox/assigners/point_assigner.py

mmdet/core/bbox/samplers/instance_balanced_pos_sampler.py

mmdet/core/bbox/samplers/iou_balanced_neg_sampler.py

mmdet/core/bbox/samplers/ohem_sampler.py

mmdet/core/bbox/samplers/pseudo_sampler.py

mmdet/core/bbox/samplers/random_sampler.py

mmdet/models/anchor_heads/free_anchor_retina_head.py

mmdet/models/anchor_heads/fsaf_head.py

mmdet/models/anchor_heads/ga_rpn_head.py

mmdet/models/anchor_heads/guided_anchor_head.py

mmdet/models/anchor_heads/rpn_head.py

mmdet/models/bbox_heads/bbox_head.py

mmdet/models/losses/cross_entropy_loss.py

mmdet/models/losses/ghm_loss.py

mmdet/ops/masked_conv/masked_conv.py