text: Title: UserWarning "this overload of nonzero is deprecated" when using with PyTorch 1.6
  Body: Hi,
  Not really a big deal, just started getting a deprecation warning after updating to the PyTorch 1.6:
  
  ```
  /opt/conda/lib/python3.7/site-packages/pytorch_metric_learning/utils/loss_and_miner_utils.py:79: UserWarning: This overload of nonzero is deprecated:
  	nonzero()
  Consider using one of the following signatures instead:
  	nonzero(*, bool as_tuple) (Triggered internally at  /opt/conda/conda-bld/pytorch_1595629427478/work/torch/csrc/utils/python_arg_parser.cpp:766.)
    a1_idx = matches.nonzero()[:, 0].flatten()
  ```
  
  I use pytorch-metric-learning==0.9.89