RNN (sru) is incompatible with torch 1.3

[kball-asapp]

Describe the bug

flambe/flambe/nn/rnn.py

Line 146 in 703c343

output, state = self.rnn(data, state, mask_pad=(-padding_mask + 1).byte())

raises this error with torch 1.3:

RuntimeError: Negation, the `-` operator, on a bool tensor is not supported. If you are trying to invert a mask, use the `~` or `logical_not()` operator instead.

Software Versions (please complete the following information):

• OS: macOS 10.14
• Python Version 3.7.5
• PyTorch Version 1.3.1
• Flambé Version 0.2.10