Problem with train_bc_rnn.py in python 3.8

[amandlek]

When running examples/train_bc_rnn.py on python 3.8, if you set config.train.hdf5_cache_mode = "low_dim" and config.train.num_data_workers = 2, the training fails. It appears to be related to an issue with global variables in utils/obs_utils.py and the torch dataloader workers not having those set properly.