Use boolean mask to avoid deprecated usage

[soraxas]

The usage of byte tensor to index torch tensor is deprecated, and will constantly output warnings on every forward pass (e.g., see this)

Example output from running the bnaf.py (the only file that had this deprecated usage):

 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 Start step 0; Training for 10000 steps:   0 % | | 47 / 10000[00:00 < 03:19, 49.78 it / s
 [W IndexingUtils.h: 30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 [W IndexingUtils.h:30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 Start step 0; Training for 10000 steps:   0 % | | 48 / 10000[00:00 < 03:19, 49.78 it / s
 [W IndexingUtils.h: 30] Warning: indexing with dtype torch.uint8 is now deprecated, please use a dtype torch.bool instead.(function expandTensors)
 ...

This PR switch it to a bool tensor instead.