BUG: Opt-level 01 not working

I have PyTorch v1.2 and Apex installed. I was able to get opt_level 00 and 03 working. But, opt_level 01 option isn't working. From the debugging I've done, looks like it's crashing at prefetcher = data_prefetcher(train_loader). However, there aren't any error messages to help with debugging.

OUTPUT:

$ python main_amp.py -a resnet50 --b 224 --workers 4 --opt-level O1 data
opt_level = O1
keep_batchnorm_fp32 = None <class 'NoneType'>
loss_scale = None <class 'NoneType'>

CUDNN VERSION: 7602

=> creating model 'resnet50'
Selected optimization level O1: Insert automatic casts around Pytorch functions and Tensor methods.

Defaults for this optimization level are:
enabled : True
opt_level : O1
cast_model_type : None
patch_torch_functions : True
keep_batchnorm_fp32 : None
master_weights : None
loss_scale : dynamic
Processing user overrides (additional kwargs that are not None)...
After processing overrides, optimization options are:
enabled : True
opt_level : O1
cast_model_type : None
patch_torch_functions : True
keep_batchnorm_fp32 : None
master_weights : None
loss_scale : dynamic

Start with main_amp.py and the data_prefetcher(train_loader) call, then reproduce the reported command with opt-level O1 and compare it with O0 and O3. Investigate why O1 stops without an error message; done means identifying the failure and adding a verified fix or a diagnostic that exposes it.